You can use Gimp to create the graphic elements for window border, and other themes, but I have no clue what dimensions they should be, what image formats can be used, where that information can be found, or how to put the graphic elements together into a working and usable theme.
Can someone please point us to a tutorial for that, if there is one?
Although I don't know about the themes, I can tell you my method for creating and using custom Icons. You can create custom Icons in Gimp, and save them as .svg files if you have the svg plugin. Save your Icons as png files with standard icon dimensions if your version of Gimp cant handle the svg format. You can use Inkscape to create svg icons, or even use it to convert and resize images created in gimp for later use as icons. I don't like Inkscape's png export, but for svg Icons it works very well. Whether you use Inkscape, Gimp, or other image creation software to make your icons is totally a matter of personal choice, just create the image to a standard Icon size, in either svg or png format. Either of those image formats can usually include transparent areas no matter which program you used to make them. I prefer Inkscape and The Gimp, because they can create lines and marks as small as ¼ pixel, and for icon creation you really need that.
To prepare your new icon for use, open the containing folder as administrator, open a new window and browse to the user/share/icons directory, or user/share/pixmaps if you prefer, and create a folder with the unique name of your intended Icon theme. Copy icons from your working folder and paste the copies into the new folder you just created, rename them. Keep the originals of your custom icons in your working folder, because any you actually use will become the property of Root. Now you can close the file manager windows.
Return to the desktop.
Find and Right click a launcher you want to change the icon for, chose "properties" from the drop down list, and click the icon image in the upper left corner of the properties window, which should open a browser window for you. Navigate to the saved copy of your new icon in the folder you created and select it. Now the browser window should close and you should see your custom made icon in the upper left corner of the properties window, close the properties window. It's tedious to assign Icons this way, but I'm not an experienced Mint User, and that's the only method for using your own custom made Icons that I know right now.
(Please be aware that I'm relatively new to Linux Mint myself, so my method is just naturally going to be a newb method. If you are reading this, and you find a mistake, or have a better way, please don't hesitate to tell me, and if you come across a good theme building tutorial for Linux Mint please post a link to it. )
Wishing there really was a Linux guide for the thick and stupid, so I could be reading it right now.