Allow user to specify background colors for layouts.

Use similar styles for Remix and Next layouts.

Make templates work with both Remix and Next.

Switch to Tailwind for improved DX.

Make the more items occupy the rest of the screen's width in
left-sidebar-with-menu layout.

Separate styles from framework implementations.

Use css-utils, which is powered by Goober, for separating HTML semantics
and styling, as compared from styled-components where semantics and
styling are defined together.