1. You can customize every widget which have assigned a ChStyle directly. If you want to customize title bar (i.e. as Chrome do on Windows) i think that this is not possible directly as it is too tied to platform(wm create window borders on X11, on Windows it is possible to customize that[something with NC_PAINT i think]). Another option is to create border-less window and paint your title bar on it and when user click and drag your window you move the window according to new coordinates received via mouse events.
4. No.
5. On Linux/BSD/Unix the minimal requirement is to have X11 and you may compile your application with only X11 support(define NOGTK flag). If you want to have theming based on gtk+ then gtk is a requirement. On Windows if uxtheme.dll is present(XP, Vista, W7) and theme is activated then Upp create the widgets theme based on informations retrieved from uxtheme, though that is not a requirement. So, on Windows only requirements are these libraries which already exist and are the base of windows(advapi32 comdlg32 comctl32). Practically there are no requirements as Upp tend to put all requirements in one exe (static linkage).