fix(themes): white editor and grey sidebar in light theme

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/index.css

@@ -550,6 +550,13 @@ fieldset:disabled :is(button, input, select, textarea) {
   cursor: not-allowed;
 }
 
+/* Workspace surfaces: the note editor and the folder sidebar. Themes may override them (the light theme
+   uses a white sheet next to a grey sidebar); by default they follow the page and panel colours. */
+:root {
+  --bg-editor: var(--bg-app);
+  --bg-sidebar: var(--bg-panel);
+}
+
 /* Outliner: indent per level and the width a row keeps for its text. Phones indent less and let very
    deep rows run wider than the screen (the note then scrolls sideways) instead of squeezing the text. */
 :root {