(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/RightPanelStyles.ts

@@ -1,13 +1,14 @@
-import {colors, vars} from 'app/client/ui2018/cssVars';
+import {theme, vars} from 'app/client/ui2018/cssVars';
 import {icon} from 'app/client/ui2018/icons';
 import {styled} from 'grainjs';
 
 export const cssIcon = styled(icon, `
   flex: 0 0 auto;
-  --icon-color: ${colors.slate};
+  --icon-color: ${theme.lightText};
 `);
 
 export const cssLabel = styled('div', `
+  color: ${theme.text};
   text-transform: uppercase;
   margin: 16px 16px 12px 16px;
   font-size: ${vars.xsmallFontSize};
@@ -23,11 +24,12 @@ export const cssRow = styled('div', `
   display: flex;
   margin: 8px 16px;
   align-items: center;
+  color: ${theme.text};
   &-top-space {
     margin-top: 24px;
   }
   &-disabled {
-    color: ${colors.slate};
+    color: ${theme.disabledText};
   }
 `);
 
@@ -46,6 +48,6 @@ export const cssButtonRow = styled(cssRow, `
 `);
 
 export const cssSeparator = styled('div', `
-  border-bottom: 1px solid ${colors.mediumGrey};
+  border-bottom: 1px solid ${theme.pagePanelsBorder};
   margin-top: 16px;
 `);