Make `$base-font-size` reflect reality

The value previously set for `$base-font-size` in `variables` was
`16px`, which is a noble idea, but the only place it’s used is in
`sidebar`, where it was immediately overridden to `12px`. So, in reality,
it’s 12px.

src/styles/sidebar/sidebar.scss

-@use '../variables' as var with (
-  $base-font-size: 12px,
-  $base-line-height: 20px
-);
+@use '../variables' as var;
 
 // Base styles
 // -----------

src/styles/variables.scss

@@ -70,7 +70,7 @@ $sans-font-family: 'Helvetica Neue', Helvetica, Arial, 'Lucida Grande',
   sans-serif !default;
 $mono-font-family: Open Sans Mono, Menlo, DejaVu Sans Mono, monospace !default;
 
-$base-font-size: 16px !default;
+$base-font-size: 12px !default;
 $base-line-height: 20px !default;
 
 $body1-font-size: 12px;