(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/components/ParseOptions.ts

@@ -1,6 +1,6 @@
 import {bigBasicButton, bigPrimaryButton} from 'app/client/ui2018/buttons';
 import {squareCheckbox} from 'app/client/ui2018/checkbox';
-import {colors, testId} from 'app/client/ui2018/cssVars';
+import {testId, theme} from 'app/client/ui2018/cssVars';
 import {cssModalButtons} from 'app/client/ui2018/modals';
 import {ParseOptionSchema} from 'app/plugin/FileParserAPI';
 import {Computed, dom, DomContents, IDisposableOwner, input, Observable, styled} from 'grainjs';
@@ -107,12 +107,18 @@ const cssParseOptionName = styled('div', `
   margin-bottom: 8px;
 `);
 const cssInputText = styled(input, `
+  color: ${theme.inputFg};
+  background-color: ${theme.inputBg};
   position: relative;
   display: inline-block;
   outline: none;
   height: 28px;
-  border: 1px solid ${colors.darkGrey};
+  border: 1px solid ${theme.inputBorder};
   border-radius: 3px;
   padding: 0 6px;
   width: 100%;
+
+  &::placeholder {
+    color: ${theme.inputPlaceholderFg};
+  }
 `);