Fixes scrollbar colors in dark mode

bookwyrm/static/css/themes/bookwyrm-light.scss

@@ -19,6 +19,8 @@ $scheme-main: $white-bis;
 $background-body: $white;
 $background-secondary: $white-ter;
 $background-tertiary: $white-bis;
+$scrollbar-track: $background-secondary;
+$scrollbar-thumb: $grey-lighter;
 
 /* highlight colors */
 $primary-highlight: $primary-light;