diff --git a/frontend/src/components/Modal.css b/frontend/src/components/Modal.css
index 78a7adb..a87c9f3 100644
--- a/frontend/src/components/Modal.css
+++ b/frontend/src/components/Modal.css
@@ -242,14 +242,6 @@
   margin-bottom: var(--sp-3) !important;
 }
 
-.custom-theme-editor {
-  background: var(--ds-surface-2);
-  border: 1px solid var(--ds-border);
-  border-radius: var(--radius);
-  padding: var(--sp-3);
-  margin-bottom: var(--sp-4);
-}
-
 .custom-theme-grid {
   display: grid;
   grid-template-columns: repeat(auto-fill, minmax(140px, 1fr));
diff --git a/frontend/src/components/ProfileModal.tsx b/frontend/src/components/ProfileModal.tsx
index fb0231e..7b8f8fd 100644
--- a/frontend/src/components/ProfileModal.tsx
+++ b/frontend/src/components/ProfileModal.tsx
@@ -13,7 +13,7 @@ import { useAuth } from '../context/AuthContext'
 import { hashIndex } from '../lib/avatar'
 import { applyTheme, DEFAULT_CUSTOM_COLORS } from '../lib/theme'
 import type { CustomTheme, CustomThemeColors } from '../types'
-import { CustomThemePreview } from './CustomThemePreview'
+import { ThemeBuilderModal } from './ThemeBuilderModal'
 import { UserAvatar } from './UserAvatar'
 import './Modal.css'
 
@@ -388,72 +388,19 @@ export function ProfileModal({ onClose }: ProfileModalProps) {
         </div>
 
         {editingTheme && (
-          <div className="custom-theme-editor">
-            <input
-              type="text"
-              className="custom-theme-name-input"
-              value={editNameDraft}
-              onChange={(e) => setEditNameDraft(e.target.value)}
-              placeholder="Theme name"
-              maxLength={50}
-            />
-            <CustomThemePreview
-              colors={editColorsDraft}
-              highlightedField={highlightedField}
-              onHighlight={setHighlightedField}
-            />
-            <div className="custom-theme-grid">
-              {CUSTOM_COLOR_FIELDS.map((field) => (
-                <label
-                  key={field.key}
-                  className={`custom-theme-field${highlightedField === field.key ? ' custom-theme-field-highlighted' : ''}`}
-                  onMouseEnter={() => setHighlightedField(field.key)}
-                  onMouseLeave={() => setHighlightedField(null)}
-                >
-                  <input
-                    type="color"
-                    value={editColorsDraft[field.key]}
-                    onChange={(e) => handleEditColorChange(field.key, e.target.value)}
-                    onFocus={() => setHighlightedField(field.key)}
-                    onBlur={() => setHighlightedField(null)}
-                  />
-                  <span>{field.label}</span>
-                </label>
-              ))}
-            </div>
-            <div className="custom-theme-scheme">
-              <span>Native controls (scrollbars, form inputs)</span>
-              <div className="custom-theme-scheme-toggle">
-                <button
-                  type="button"
-                  className={`btn-secondary${editColorsDraft.color_scheme === 'light' ? ' custom-theme-scheme-active' : ''}`}
-                  onClick={() => handleEditColorChange('color_scheme', 'light')}
-                >
-                  Light
-                </button>
-                <button
-                  type="button"
-                  className={`btn-secondary${editColorsDraft.color_scheme === 'dark' ? ' custom-theme-scheme-active' : ''}`}
-                  onClick={() => handleEditColorChange('color_scheme', 'dark')}
-                >
-                  Dark
-                </button>
-              </div>
-            </div>
-            <div className="modal-actions">
-              <button type="button" className="btn-secondary" onClick={closeEditor}>
-                Cancel
-              </button>
-              <button
-                type="button"
-                className="btn-primary"
-                onClick={handleSaveThemeEdit}
-                disabled={savingThemeEdit}
-              >
-                {savingThemeEdit ? 'Saving…' : 'Save'}
-              </button>
-            </div>
-          </div>
+          <ThemeBuilderModal
+            colorFields={CUSTOM_COLOR_FIELDS}
+            name={editNameDraft}
+            onNameChange={setEditNameDraft}
+            colors={editColorsDraft}
+            onColorChange={handleEditColorChange}
+            highlightedField={highlightedField}
+            onHighlight={setHighlightedField}
+            error={themeError}
+            saving={savingThemeEdit}
+            onSave={handleSaveThemeEdit}
+            onCancel={closeEditor}
+          />
         )}
 
         <hr className="modal-divider" />
diff --git a/frontend/src/components/ThemeBuilderModal.css b/frontend/src/components/ThemeBuilderModal.css
new file mode 100644
index 0000000..423a089
--- /dev/null
+++ b/frontend/src/components/ThemeBuilderModal.css
@@ -0,0 +1,22 @@
+.theme-builder-modal {
+  width: min(820px, 95vw);
+  max-height: 88vh;
+}
+
+.theme-builder-layout {
+  display: grid;
+  grid-template-columns: minmax(0, 1fr) minmax(0, 1fr);
+  gap: var(--sp-5);
+  align-items: start;
+  margin-bottom: var(--sp-2);
+}
+
+.theme-builder-preview-col .custom-theme-preview {
+  margin-bottom: var(--sp-4);
+}
+
+@media (max-width: 680px) {
+  .theme-builder-layout {
+    grid-template-columns: 1fr;
+  }
+}
diff --git a/frontend/src/components/ThemeBuilderModal.tsx b/frontend/src/components/ThemeBuilderModal.tsx
new file mode 100644
index 0000000..7d6cac6
--- /dev/null
+++ b/frontend/src/components/ThemeBuilderModal.tsx
@@ -0,0 +1,119 @@
+import type { CustomThemeColors } from '../types'
+import { CustomThemePreview } from './CustomThemePreview'
+import './Modal.css'
+import './ThemeBuilderModal.css'
+
+interface ColorField {
+  key: keyof Omit<CustomThemeColors, 'color_scheme'>
+  label: string
+}
+
+interface ThemeBuilderModalProps {
+  colorFields: ColorField[]
+  name: string
+  onNameChange: (name: string) => void
+  colors: CustomThemeColors
+  onColorChange: (key: keyof CustomThemeColors, value: string) => void
+  highlightedField: keyof CustomThemeColors | null
+  onHighlight: (field: keyof CustomThemeColors | null) => void
+  error: string | null
+  saving: boolean
+  onSave: () => void
+  onCancel: () => void
+}
+
+// Same editor as ProfileModal.tsx used to render inline, pulled out into its
+// own wider dialog (#46) -- the profile modal's .modal is capped at
+// min(380px, 100%), which cramped the CustomThemePreview mockup that's
+// supposed to make the color-to-UI mapping easy to see. Nested on top of
+// ProfileModal rather than replacing it, same stacked-dialog pattern as
+// ImageLightbox/FilePreviewModal opening over MessageList.
+export function ThemeBuilderModal({
+  colorFields,
+  name,
+  onNameChange,
+  colors,
+  onColorChange,
+  highlightedField,
+  onHighlight,
+  error,
+  saving,
+  onSave,
+  onCancel,
+}: ThemeBuilderModalProps) {
+  return (
+    <div className="modal-scrim" onClick={onCancel}>
+      <div className="modal theme-builder-modal" onClick={(e) => e.stopPropagation()}>
+        <div className="modal-header">
+          <h2>Edit theme</h2>
+          <button type="button" className="modal-close" onClick={onCancel} aria-label="Close">
+            &times;
+          </button>
+        </div>
+
+        <div className="theme-builder-layout">
+          <div className="theme-builder-preview-col">
+            <input
+              type="text"
+              className="custom-theme-name-input"
+              value={name}
+              onChange={(e) => onNameChange(e.target.value)}
+              placeholder="Theme name"
+              maxLength={50}
+            />
+            <CustomThemePreview colors={colors} highlightedField={highlightedField} onHighlight={onHighlight} />
+            <div className="custom-theme-scheme">
+              <span>Native controls (scrollbars, form inputs)</span>
+              <div className="custom-theme-scheme-toggle">
+                <button
+                  type="button"
+                  className={`btn-secondary${colors.color_scheme === 'light' ? ' custom-theme-scheme-active' : ''}`}
+                  onClick={() => onColorChange('color_scheme', 'light')}
+                >
+                  Light
+                </button>
+                <button
+                  type="button"
+                  className={`btn-secondary${colors.color_scheme === 'dark' ? ' custom-theme-scheme-active' : ''}`}
+                  onClick={() => onColorChange('color_scheme', 'dark')}
+                >
+                  Dark
+                </button>
+              </div>
+            </div>
+          </div>
+
+          <div className="custom-theme-grid theme-builder-grid">
+            {colorFields.map((field) => (
+              <label
+                key={field.key}
+                className={`custom-theme-field${highlightedField === field.key ? ' custom-theme-field-highlighted' : ''}`}
+                onMouseEnter={() => onHighlight(field.key)}
+                onMouseLeave={() => onHighlight(null)}
+              >
+                <input
+                  type="color"
+                  value={colors[field.key]}
+                  onChange={(e) => onColorChange(field.key, e.target.value)}
+                  onFocus={() => onHighlight(field.key)}
+                  onBlur={() => onHighlight(null)}
+                />
+                <span>{field.label}</span>
+              </label>
+            ))}
+          </div>
+        </div>
+
+        {error && <p className="modal-error">{error}</p>}
+        <div className="modal-actions">
+          <button type="button" className="btn-secondary" onClick={onCancel}>
+            Cancel
+          </button>
+          <button type="button" className="btn-primary" onClick={onSave} disabled={saving}>
+            {saving ? 'Saving…' : 'Save'}
+          </button>
+        </div>
+      </div>
+    </div>
+  )
+}