The content of custom.css can be pasted into the Custom CSS field of the page you find following Settings/Administration/Server Settings/Appearance.

This style fills in a prettier way the bottom part of the left column.

It also fixes some shortcomings of the "advanced" view:

• content stuck to the left side of the window
• too thin columns even when there's a lot of room
• hidden columns when the window isn't wide enough

Here are a few pictures to compare:

Standard Style in a wide window:

Mastodys Style in a wide window:

(columns are wider, and the whole is centered)

Standard Style in a medium window:

Mastodys Style in a medium window:

(the rightest column isn't cut or hidden)

Overall, this is a very small change. I take suggestions for more improvements but I'd rather not change more than what's usefull.