Tonight I got sick of one tiny detail in the default Ubuntu theme.
First some background: I use the task panel a lot not only to select another application but also to minimize the task/application currently in focus. I do this because I think it is easier (and it *is* easier, read up on Fitt's Law for more details) to click the quite large task icon then it is to click the minimize button of the current window.
Now, the problem is that in the current default theme (at least I think it is the default, it is dark brow/grey with some purple stuff here and there and orange icons) you cannot easily and quickly see which of the tasks in the task panel is the selected one, if any. If you look carefully you will of course see it, but the difference in look/color is not much from the unselected ones. So I thought maybe I could figure out how to change it.
I was prepared for a long journey but just a few "locate" and "find" I was able to correctly guess what images that are used, and they are placed under /usr/share/themes/Ambiance/gtk-2.0/apps/img. The image in question is panel-button-active.png. I used sudo, chgrp and chmod to grant myself write access to that image (could have done the editing using sudo as well but I rather like to use my normal user) and then using The Gimp I simply filled the image with a new color that matched one of the purple ones already in use. I then had to force a reload of the panel and it was easy: $ gnome-panel --replace (you don't need to use sudo here). Voila! My new image is now used for the active task and it is much easier to see which task is active. Granted, it's not as nice looking but for me it is worth more to see it clearly.
After that change I discovered that I also wanted to change the hover image so that the purple image was not replaced by a brown/grey one. That one is named panel-button-hover.png and I used the same process as above to change that.
I have included the two changed images in this post in case you want to try it out too.