layout: fix footer font-size on large screens #496

The footer font-size media query used `var(--main-breakpoint)` in its
condition. Custom properties are invalid inside media queries, so the
whole block was silently dropped and the footer never got its intended
0.9em on large screens. Inline the 60em literal (matching the other
min-width breakpoints in this file) and drop the now-unused variable.

Author: KangaZero

static/css/main.css

@@ -1,6 +1,4 @@
 :root {
-  --main-breakpoint: 60em;
-
   --sans-serif-fonts: var(--bs-font-sans-serif);
 
   /* Colors */
@@ -487,7 +485,7 @@ footer {
   padding: 1em 0;
 }
 
-@media (min-width: var(--main-breakpoint)) {
+@media (min-width: 60em) {
   footer {
     font-size: 0.9em;
   }