In the screenshots on winsteps website, they show the browser windows and some other ones sporting the skins, but when I use nextstart, only the dekstop and the "start" menu use skins. My windows look like plain ol' windows xp windows, which is a bit jarring. I'm using the demo, with plans to register.

Winstep applications only skin themselves. To skin Explorer windows you will need either WindowBlinds or StyleXP, which is what most Winstep users also run to get the 'complete' look.

and where can i find themes to match the winstep ones

On the Winstep site, click on the Themes link at the top. This will give you access to the Winstep themes libraries of a bunch of skin repository sites. In these skin sites, look for matching StyleXP or WindowBlinds skins.