(core) Add dark mode for comments

Summary:
Also fixes the CSS for the page/workspace input so that it's always
readable in dark mode.

Test Plan: Tested manually.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3682

app/client/ui/TopBar.ts

@@ -118,6 +118,7 @@ function buildShowDiscussionButton(pageModel: DocPageModel) {
   return cssHoverCircle({ style: `margin: 5px; position: relative;` },
     cssTopBarBtn('Chat', dom.cls('tour-share-icon')),
     cssBeta('Beta'),
+    hoverTooltip('Comments', {key: 'topBarBtnTooltip'}),
     testId('open-discussion'),
     dom.on('click', () => pageModel.gristDoc.get()!.showTool('discussion'))
   );