(core) Improve dark mode

Summary:
Enhances dark mode support for the formula editor, and adds support to
the color select popup. Also fixes some bugs with dark mode.

Test Plan: Tested manually.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3847

app/client/components/GristDoc.ts

@@ -182,6 +182,8 @@ export class GristDoc extends DisposableWithEvents {
   // Holder for the popped up formula editor.
   public readonly formulaPopup = Holder.create(this);
 
+  public readonly currentTheme = this.docPageModel.appModel.currentTheme;
+
   private _actionLog: ActionLog;
   private _undoStack: UndoStack;
   private _lastOwnActionGroup: ActionGroupWithCursorPos|null = null;