(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/ui/cssInput.ts

@@ -1,12 +1,18 @@
-import {colors, vars} from 'app/client/ui2018/cssVars';
+import {theme, vars} from 'app/client/ui2018/cssVars';
 import {styled} from 'grainjs';
 
 export const cssInput = styled('input', `
+  color: ${theme.inputFg};
+  background-color: ${theme.inputBg};
   height: 30px;
   width: 100%;
   font-size: ${vars.mediumFontSize};
   border-radius: 3px;
   padding: 5px;
-  border: 1px solid ${colors.darkGrey};
+  border: 1px solid ${theme.inputBorder};
   outline: none;
+
+  &::placeholder {
+    color: ${theme.inputPlaceholderFg};
+  }
 `);