Extract input style to common CSS and remove raw uses of var(--x).

The mixin handles the fallback gracefully on browsers where it's not
supported.

assets/scss/variables.scss

@@ -32,6 +32,8 @@ $code-lineno-color: grey;
 $code-comment-color: grey;
 $code-border: $standard-border-width solid $code-border-color;
 
+$input-padding: 0.5rem;
+
 $sidenote-padding: 1rem;
 $sidenote-highlight-border-width: .2rem;