(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/client/ui2018/editableLabel.ts

@@ -9,7 +9,7 @@
  *
  * TODO: Consider merging this into grainjs's input widget.
  */
-import { colors } from 'app/client/ui2018/cssVars';
+import { theme } from 'app/client/ui2018/cssVars';
 import { dom, DomArg, styled } from 'grainjs';
 import { Observable } from 'grainjs';
 import noop = require('lodash/noop');
@@ -45,7 +45,7 @@ export const cssLabelText = styled(rawTextInput, `
 export const cssTextInput = styled('input', `
   outline: none;
   height: 28px;
-  border: 1px solid ${colors.darkGrey};
+  border: 1px solid ${theme.inputBorder};
   border-radius: 3px;
   padding: 0 6px;
 `);
@@ -97,7 +97,7 @@ export function editableLabel(label: Observable<string>, options: EditableLabelO
 
 /**
  * Provides a text input element that pretty much behaves like the editableLabel only it shows as a
- * regular input within a rigid static frame. It takes in an observable that is setf on Enter or loss
+ * regular input within a rigid static frame. It takes in an observable that is set on Enter or loss
  * of focus. Escape cancels editing. Validation logic (if any) should happen in the save function,
  * to reject a value simply throw an error, this will revert to the the saved one.
  */