You can add backdrops yourself in wb prefs. If you don't have a GFX card then you have to think about adapting images to suit your screenmode. I'd recommend using PPaint which is available off of Aminet, and has a good set of image manipulation tools, particularly for colour reduction.
Using the newicons patch will give you access to hundreds of different sets of colourful icons, similar to the 3.5/3.9 icons (although you can't use 3.5+ icons). Again this is available from Aminet.
If you want to change the way Windows and gadgets look then a program called VisualPrefs is what you're after. It doesn't have the most intuitive GUI, but with experimentation you can get good results. (also on Aminet)
Lastly, a program called fullpalette gives you control over the exact colours on workbench and is useful to use along-side visualprefs.
Hopefully that is a start. One word of warning though, beauty comes at a price and in this case, use this software *can* affect stability and performance. I never experienced any, but your milage may vary.
