It would help if you specify what you’re trying to reduce exactly. Are we just talking about the white space between topics in the topic list? You can change the default view, or this:
to this:
By adding this CSS
.topic-list td {
padding: 2px 5px;
}
and see if that gets you closer to what you want. If so, you can make that into a selectable theme as @notriddle pointed out above.
If that’s too compact for your taste you can increase the first number to add more white space.
the default is 12px 5px