(core) Add config to include custom CSS

Summary:
Adds a new environment variable that allows for custom
CSS to be included in all core static pages.

Test Plan: Tested manually in grist-core.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3419

app/client/ui2018/cssVars.ts

@@ -110,6 +110,7 @@ export const vars = {
   controlBorderRadius:  new CustomProp('border-radius', '4px'),
 
   logoBg: new CustomProp('logo-bg', '#040404'),
+  logoSize: new CustomProp('logo-size', '22px 22px'),
   toastBg: new CustomProp('toast-bg', '#040404'),
 };