Fix options page CSS in Firefox

The options page CSS didn't work correctly in Firefox, since it was
using styles originally developed by the Chromium team for Chrome. This
CL fixes this by using CSS properties also available in Firefox.

This CL also sets explicitly a font family and size, since this makes
the options page look better in Firefox (specially the experiments
page).

Fixed: twpowertools:119
Change-Id: I49c7a52c02b9ddd462d95da4148ebd893c87234f
diff --git a/src/static/options/options.css b/src/static/options/options.css
index 68057ce..39783ef 100644
--- a/src/static/options/options.css
+++ b/src/static/options/options.css
@@ -1,5 +1,7 @@
 body {
   padding-top: 16px;
+  font-family: 'Roboto', -apple-system, BlinkMacSystemFont, 'Segoe UI Adjusted', 'Segoe UI', 'Liberation Sans', sans-serif;
+  font-size: 13px;
 }
 
 main {