There has been alot of talk of how to get minimap button borbers changed. It is indeed done by simple graphic replacement. You need to put 2 files to your wowfolder/interface/minimap folder (this folder does not exist by default, you have to make it). The files are minimap-trackingborder.blp (the actual borders file) and ui-minimap-zoombutton-highlight.blp (graphic file for highlight when u hover your cursor over a minimap button). These BLP files are encoded .PNG graphic files.
Below on a link you can find border replacement I have made for myself. Feel free to use those if you want or edit them to suit your own liking.
To edit the files you need to decode them with a program. Wow Image converter is a simple program that can decode and encode the files (BLP <-> PNG). After decoding the files you can edit them with any graphic editor program. After you have edited the pictures you need to encode them back to BLP format with the converter, then just copy the files to the right folder and it's finished.
Pic of the border replacement I made for myself and that's included within the link below. The blue cube over one of the borders is the highlight graphic I made to go with the border. (Tried to make it look like VolM skin from cycircled, it aint perfect and i will prolly try to improve it a bit still.)
http://files.filefront.com/minimap+b.../fileinfo.html
Pack with the graphic replacement files i made.
Bad sides on this is that not all addons use the borders but their own borders. Deadly boss mods for example doesn't seem to use the border graphic and it's borders stayed as the default, but it still uses the highlight graphic. So these kind of problems may appear. Also it seems that diffirend addons minimap icons align the borders a bit diff. So if i compare the highlight graphic for example it might appear a pixel more to the right then others. But in overall it works kinda nice and can make a nice finishing touch to your customized UI.
Hope this helps a bit. Making the graphics aint hard, but it takes a bit of tweaking to get things alligned etc. This might not be the only way, but it's one way of doing it. Good thing with this way is that you dont need any kind of addon to do this, so it's good for people who try to keep their addons and memory usage in the minimum.
-Carud