Hi Joe,
Other than a simplistic manual under the help there is really nothing on the how to's of editing graphical items in a theme. However, to do some real editing to the graphics for SONAR or CbB you must use a graphic editor (or a vector graphics editor), like Adobe Illustrator, Corel Draw, etc. There are some free editors like GIMP
https://www.gimp.org/ and inkscape
https://inkscape.org/ . I have used both, but find inkscape to be more advanced whereas GIMP is simple to use. It's just a matter of learning how to use their tools. Inkscape has a third party manual, but it is not exhaustive. With your editor of choice incorporated into Theme Editor, you can export graphic items to your graphics editor. There you can make whatever changes as complex or simple as you like (or to the best of your abilities and knowledge of your editor), like swapping colors or making new icons on the buttons or even changing the shape of knobs (which can also be downloaded from other websites) as long as your new graphics fit in the prescribed space. So, other than just changing colors on a few items, Theme Editor is merely a means of consolidating your ideas on a theme into a single cohesive graphic user interface in SONAR or CbB, but not to be under appreciated. I wish I could remember Theme Editor's creator and give him well deserved credit for doing this for us. I thinks it is Ben, but not sure.
It can be daunting to learn these graphic editors and even more so when it comes to actually making detailed changes to the graphics in SONAR/CbB, not to mention the large number of items available. I have done three themes in the past and it does require time, creativity and a good amount of trial and error (and a huge amount of patience) if you really want to get into the nitty gritty of "pixel flipping". It's not for the faint of heart...ok I got a little dramatic there. It is also fun and in the end pleasing to see things in SONAR the way you like them, particularly if you spend as much time working with it as I have or the many others here.
I wish I could offer some help along these lines, but given your occupation, it's probably going to be a cakewalk for you. Feel free to PM me if you have any questions.
Kind regards,
tecknot