It is certainly fun to play with the existing styles, but creating one from scratch seems pretty complex. Not nearly as complicated as WindowBlinds, though, so that's a plus

It looks like it might be possible to build a convincing Windows 95/Classic/Chicago theme with this.
Here is a list of UI bugs/suggestions:
- Tab order in some of the the [Edit Style] sections is broken. Go to [Shadows], [Colors], or [Advanced], select a field, hit tab a couple of times, and focus disappears completely.
- Combo box behaviour in the [Edit Style]->[Advanced] section is weird; if this is a custom control, could you switch to using the standard Windows one? When you click on the style combobox, the currently-selected item is displayed at the very top of the list (instead of being under the mouse pointer) which makes it hard to jump to the next/previous entry in the list. I know you can do that by hovering over the box and using the scroll wheel, but sometimes I want to see my choices.
- The [Edit Style]->[Colors]->[Select ### Color] interface is weird. Clicking one of those buttons essentially displays a modal dialog box, but it's not styled to look like one. Why not just make the box disappear when you click away from it instead of making the user click the [close] button?
- [Styles] section scrollbar acts weird. No left/right arrows, no hilight on hover, jump scrolling as you click+drag it. It actually took me a long time to even realize there were more than 9 available styles because I didn't know the list was scrollable. Could this be converted to a standard Windows control?
- It would be nice if the Minimize/Maximize/Close buttons were included in the image previews for the styles, so that you could easily tell which ones had custom buttons.
- There are no tooltips/descriptions for the two icons in the corner of the style preview images. One is probably "custom background", but not sure what the color wheel means.
We will check the tab order, it is still in flux so tab ordering has not been set fully yet.
The combo box is a standard control. The box will show the currently selected item at the top of the list and this will typically appear under the control (unless there is no room). Standard combo boxes do not show the selected item directly under the mouse. As the list is very large, if you do not have room under the box then the list would appear above which might be the issue you are seeing here?
If you click away from the select ### colour box it does indeed close automatically. The close button is there as a convenience.
The scrollbars in app do not have a mouseover state that is true. The default OS scrollbar is unfortunately not a good fit UI wise which is why we have our own. I will see what we can do to tweak it.
Agree on the min/max/close buttons. Styles can now include their own mini preview so they can do what they wish with them (Arrow on the online tab uses one once installed).
The first icon is indeed custom background, the second is recolourable.