Add 'selected' background/foreground colors to the theme css variables

[jasongrout]

Showing a selection in a list is coming up fairly often (#5364, #1095, etc.). Perhaps we should just have a theme variable for a selected item's background and foreground color.

[jasongrout]

Right now in the filebrowser and now completions (after #5346), we use:

background: var(--jp-brand-color1);
color: white;