Hey Jeff! I'm liking the progress your making on this, I just have a tiny nitpick.
Shouldn't this stick to the length units you were using everywhere else?
In this case:

font-size: 120%; --> font-size: 1.2em;

Redesign is coming along nicely!

We don't use named HTML colours; instead, we use hex values.

Is this the yellow at the top of the admin page? This should probably be a constant in defs.less.

You probably already realize this but we will want some way of making the other tabs .active.

I don't know how old this code is, but we should probably update from using view import paths to view names in the {% url %} tags here.
If that isn't a thing we can do easily, don't worry about it.

This property is out of alphabetical order. I know you didn't add it, but could you fix that?

calc(@admin-sidebar-width - 1.5em)
Fun fact: LESS will evaluate calc() statements so they are constants in the resulting CSS file.

This is a repeated rule (.module has it already).

.module.recent-actions should be specific enough to override .module without !important, unless something else is overriding it?

This should be either border-top: none or border-top-width: 0px.

Can you format as &:first-of-type > h2 ? It makes it more readable.

While you're here, can you format this asfieldset > h2? Much more readable.

calc(@admin-sidebar-width - 0.1em