Use only relative font sizes (rust-lang#894)

This replaces the only use of px for font-sizes by setting up a base
rem size on the root element in a way that is easy to calculate (1 rem =
10px) and scaling up according to browser settings.

src/theme/css/chrome.css

@@ -87,7 +87,7 @@ html:not(.sidebar-visible) #menu-bar:not(:hover).folded > #menu-bar-sticky-conta
 .menu-title {
     display: inline-block;
     font-weight: 200;
-    font-size: 20px;
+    font-size: 2rem;
     line-height: var(--menu-bar-height);
     text-align: center;
     margin: 0;

src/theme/css/general.css

@@ -2,6 +2,11 @@
 
 @import 'variables.css';
 
+:root {
+    /* Browser default font-size is 16px, this way 1 rem = 10px */
+    font-size: 62.5%;
+}
+
 html {
     font-family: "Open Sans", sans-serif;
     color: var(--fg);
@@ -11,7 +16,7 @@ html {
 
 body {
     margin: 0;
-    font-size: 1rem;
+    font-size: 1.6rem;
     overflow-x: hidden;
 }