Hi Ben,
You cannot use a background image, as the image will not become included in the CSS (the Style Sheet code the template is "made of"), even if you enter it in the Theme Settings.
A few templates, like subSilver, allow you to do
some changes of colours, font size etc. Most templates allow no changes at all of that kind.

If you want to experiment with that:

Make sure to have another template installed, to switch to, if you make a mistake.

Don't make two contrasting colours the same or similar; like black text on black/dark background, for example...

Stay in the Admin Panel, and open the forum index page in a new window/tab, to check out the changes, while working.
If you want to do something more advanced than that, you'll need to make your own CSS (if you have the knowledge):
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Then again, there are more than 300 Style Templates to choose from...
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Last but not least:
Please use the forums in the Support section (where I have moved this now) for questions.
FoxLeonard