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/style.scss

@@ -78,6 +78,10 @@ h1, h2, h3, h4, h5, h6 {
   }
 }
 
+.input[type="text"], textarea {
+  @include textual-input;
+}
+
 nav {
   width: 100%;
   margin: 0rem 0rem 1rem 0rem;