Actually you can use a browser plugin called GreaseMonkey for FireFox or TamperMonkey for Chrome
With these plugins, you can set up scripts to run for specific websites in your browser that change the look and feel to anything
you want, without affecting how anyone else sees the site.
Basically they layer on changes onto a page after downloading them on to your computer from the website but before fully displaying them to you (that's "client side" for the nerds), instead of having the website itself change for everyone (i.e. "server side") or have the website itself provide themes.
All you would need is a look and feel CSS StyleSheet that you like, and set it up to load for mmaforum. Depending on how MMAForum is styled, you could use a generic one that you find on google or have someone with web knowledge whip up a custom one.
Here's an example of what these plugins are capable of, for facebook for example: