(core) Add dark mode to user preferences

Summary:
Adds initial implementation of dark mode. Preferences for dark mode are
available on the account settings page. Dark mode is currently a beta feature
as there are still some small bugs to squash and a few remaining UI elements
to style.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Subscribers: paulfitz, jarek

Differential Revision: https://phab.getgrist.com/D3587

app/server/lib/ICreate.ts

@@ -1,3 +1,4 @@
+import {getThemeBackgroundSnippet} from 'app/common/Themes';
 import {Document} from 'app/gen-server/entity/Document';
 import {HomeDBManager} from 'app/gen-server/lib/HomeDBManager';
 import {ExternalStorage} from 'app/server/lib/ExternalStorage';
@@ -98,11 +99,12 @@ export function makeSimpleCreator(opts: {
       }
     },
     getExtraHeadHtml() {
-      let customHeadHtmlSnippet = '';
+      const elements: string[] = [];
       if (process.env.APP_STATIC_INCLUDE_CUSTOM_CSS === 'true') {
-        customHeadHtmlSnippet += '<link rel="stylesheet" href="custom.css">';
+        elements.push('<link rel="stylesheet" href="custom.css">');
       }
-      return customHeadHtmlSnippet;
+      elements.push(getThemeBackgroundSnippet());
+      return elements.join('\n');
     },
   };
 }

app/server/lib/sendAppPage.ts

@@ -55,6 +55,7 @@ export function makeGristConfig(homeUrl: string|null, extra: Partial<GristLoadCo
     survey: Boolean(process.env.DOC_ID_NEW_USER_INFO),
     tagManagerId: process.env.GOOGLE_TAG_MANAGER_ID,
     activation: getActivation(req as RequestWithLogin | undefined),
+    enableCustomCss: process.env.APP_STATIC_INCLUDE_CUSTOM_CSS === 'true',
     ...extra,
   };
 }