I guess without knowing what your looking for, it's hard to make suggestions! (domain name mythemes.org appears to be for sale, and is not an actual site from what I can tell) This looks like a really easy theme to use though
https://freevps.us/thread-19052.html this one is cool looking too
https://community.mybb.com/mods.php?action=view&pid=592
To change the color of any part of your mybb, first use a color picker tool such as
https://chrome.google.com/webstore/detai...dhjedfccka or if your working offline, there are several free desktop tools that do the same thing.
Once you have the html code for the color you can do a search in your css files (just paste the color code in your search box). for example the blue in the thread header here is #3796db and the search will show you anything that is that color, and you can manually change it to any other color you want by just replacing the color code, save, then refresh (
like if you wanted this color for your header, you would replace the blue code with #cc66ff) . This should work with any "flat" theme that doesnt use transparency (might work for that too, don't honestly know). This trick will also help you find pesky "hidden" elements in your css, as long as they have a visible color. When your designing elements in PS, mark down the color codes of key items like your logo so you can use those colors elsewhere to color coordinate.
Your also gonna find that inspect element is your friend (you prob already knew that though). Say for example this was my site, and I forgot where my header image was, I can inspect and see that it's "back.png" under "icy", inside the header div (which can help find stuff in css too. By searching for back.png I could find the image properties and change the image size, or from % to pixels in css).