Mine is a slideshow of all the repaints done thus far from the MWO forums. + the meme pic of the innersphere.
It's my parents comp...so I don't get a say in background... otherwise, it might be this: ....or this....
Dihm, www.virtualcustoms.net. What me and Grim are using are paid, premium themes called Apocalypse. They're only about 5 bucks and very well worth the money. I kept the files just in case any of my friends showed an interest in them, but I also buy themes from them pretty regularly as a way to support what they do.
I am using a custom theme from that site, but mine isn't Apocalypse. I'm using HUD Red. They are similar though.
All I do is change it back to normal Windows Aero when we play a game, and change it back after. MegaMek Lab works fine with it on or off. I'd say it's only about a 10 second inconvenience.
There are 86,400 seconds in a day. Assuming that your inconvenience takes 10 seconds to switch it both to and from your theme that means that in a mere 4,320 games of megamek you will have squandered an entire day of your life switching themes. Doesn't seem worth it to me.