KDE-Forum.org

Widgets are changed in KDE through the Style module in KControl (KControl > Appearance and Themes > Style). For each style, there's a corresponding preview of what the widgets would look like.

If you want to look for more styles, you can check out http://www.kde-look.org in the Themes/Styles sections.

Sorry I couldn't get back to you ASAP. I'm not sure about this, but I think that you need to do a bit of programming to make widgets, because widgets are more like mini-programs than just images. So I guess you'd have to learn a bit of KDE programming and Qt (which is the toolkit used by KDE) to do that. But, like I said, I'm not too sure.

Btw, just some clarification of some terms used in KDE

widgets - these are the tabs, buttons, progress bars, drop down menus, etc.
style - the appearance of widgets, can also be called widget style
theme - over-all appearance of the whole desktop, which includes the desktop background, color schemes, (widget) styles, icons, fonts, and screensavers.