hterm: fix overlay colors
hterm: fix overlay colors

Fix the css vars to use rgb(var(...)), and also add css var for
font-size.

Do not show overlays during initialization until prefs are loaded.

Bug: 1083055
Change-Id: Ibfb06e78240990fae500204c5b877f852b986894
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/2203738
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm