🐛(frontend) fix editor canvas background and dropdown check in dark mode

Setting the BlockNote CSS var lost to BlockNote's own
.bn-root[data-color-scheme=dark] rule, so the editor kept Mantine's dark
surface. Set the background directly on .bn-root .bn-editor (which outranks
BlockNote's .bn-editor rule) so the canvas shares the page background in any
theme. Also make the dropdown selected-check use the theme-aware neutral text
colour instead of a fixed gray that was invisible on the dark menu.

Signed-off-by: MashdorDev <dorzairi@ymail.com>

Author: MashdorDev

src/frontend/apps/impress/src/components/dropdown-menu/DropdownMenu.tsx

@@ -287,7 +287,7 @@ export const DropdownMenu = ({
                   <Icon
                     iconName="check"
                     $size="20px"
-                    $theme="gray"
+                    $color="var(--c--contextuals--content--semantic--neutral--primary)"
                     aria-hidden="true"
                   />
                 )}

src/frontend/apps/impress/src/features/docs/doc-editor/styles.tsx

@@ -8,9 +8,6 @@ export const DocsEditorStyle = createGlobalStyle`
   * Token Blocknote
   */
   .bn-root[data-color-scheme] {
-    /* Let the editor share the page background instead of BlockNote's own
-       (Mantine) light/dark surface, so the canvas matches the app in any theme. */
-    --bn-colors-editor-background: transparent;
     --bn-colors-editor-text: var(
       --c--contextuals--content--semantic--neutral--primary
     );
@@ -24,8 +21,11 @@ export const DocsEditorStyle = createGlobalStyle`
   }
 
   .bn-root {
+    /* Editor shares the page background rather than BlockNote's Mantine
+       surface, so the canvas matches the app background in any theme. */
     .bn-editor {
       height: 100%;
+      background-color: transparent;
     }
 
     .mantine-Menu-itemLabel,