Add proper completion info styles and preview

### What should we do?

Autocompletion should look good in both styles, and also be easily previewed in the demo project.

### Why should we do it?

So that we can use this theme with autocompletion.

Start by inspecting the existing theme styles and the demo project, then locate how autocompletion completion information is rendered in each style. Done means autocompletion looks appropriate in both styles and the result can be previewed in the demo project.

Written by the indexing model from the issue text.