The more I use SeaLion instead of ArcticFox, the more I start to like it. I think that the browser is actually faster than ArcticFox. I have now found a way to make SeaLion use the gtk theme of the system, in our case the Mint theme.
To make this happen one has to download the addition pack, which can be found at the homepage of SeaLion on github, as maintained by wicknix. Click “Git Repository” in the browser in the top. Then choose Repositories and then SeaLion. Then go to SeaLion-32.4.1 and you will find SeaLion-Extras.zip. Download this file and under Themes you will find gnomerunner-linux.xpi. Drag this file into SeaLion and it will install this theme. Enable the theme and restart SeaLion. You will now be greeted with a much nicer look.