[MM3-users] custom css?

I've installed the mailman3, mailman3-full, mailman3-web, and python3-django-mailman3 packages on Debian trixie. Everything is working - thank you developers!

However, I would like to modify how the web pages look (both postorius and hyperkitty). I noticed that the web interface on this server (lists.mailman3.org) looks much nicer than the one showing on my server. For one, on my installed instance, the left and right margins are not set, so content is flush with the edge of the screen.

What is the easiest/most maintainable way to add my own css code and/or files to adjust how the web interface is displayed? Is that possible?

Thank you! jamie