(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/lib/koForm.css

@@ -108,7 +108,7 @@ div:hover > .kf_tooltip {
   box-shadow: 0 1px 1px 1px rgba(0,0,0,0.15);
   line-height: 1.1rem;
   font-size: 1rem;
-  color: #606060;
+  color: var(--grist-theme-prompt-fg, #606060);
   z-index: 10;
 }
 

app/client/lib/pausableObs.ts

@@ -0,0 +1,41 @@
+import {IDisposableOwner, Observable} from 'grainjs';
+
+export interface PausableObservable<T> extends Observable<T> {
+  pause(shouldPause?: boolean): void;
+}
+
+/**
+ * Creates and returns an `Observable` that can be paused, effectively causing all
+ * calls to `set` to become noops until unpaused, at which point the last value
+ * passed to set, if any, will be applied.
+ *
+ * NOTE: It's only advisable to use this when there are no other alternatives; pausing
+ * updates and notifications to subscribers increases the chances of introducing bugs.
+ */
+export function createPausableObs<T>(
+  owner: IDisposableOwner|null,
+  value: T,
+): PausableObservable<T> {
+  let _isPaused = false;
+  let _lastValue: T | undefined = undefined;
+  const obs = Observable.create<T>(owner, value);
+  const set = Symbol('set');
+  return Object.assign(obs, {
+    pause(shouldPause: boolean = true) {
+      _isPaused = shouldPause;
+      if (shouldPause) {
+        _lastValue = undefined;
+      } else if (_lastValue) {
+        obs.set(_lastValue);
+        _lastValue = undefined;
+      }
+    },
+    [set]: obs.set,
+    set(val: T) {
+      _lastValue = val;
+      if (_isPaused) { return; }
+
+      this[set](val);
+    }
+  });
+}