I'm not good in graphics too (at least I tell that to myself
), but for simple rectangles with some border you have the skills trust me. Also making guis in TWL is a lot of using the Theme Editor to do some theming for your own widgets you WILL have. I would start with some easy rectangles like in my theme and look how it works. The Eforen Theme MatthiasM provides with TWLExamples shows how powerful you can work with TWL (glowing outside of widget bounds etc.).
It all comes down to one question, for what do you intend to use it? A real game? Or just debugging?