I am trying out tiling window managers like scrotWM, and have noticed that GUI programs start with a "theme" (or lack, thereof) that looks like the bland Redmond theme. Is there a way to specify the appearance/themes of windows in a tiling window manager? Or is it window manager OR GUI toolkit specific? Is there anything I can do? Thanks.
Yes, theming is specific to each particular application (window manager or otherwise). Applications that use the same GUI toolkit usually use the same configuration mechanism to select a theme, so they'll look the same.
Many tiling window managers are designed to have a low visible footprint, so there's not much room for visual theming. Maybe that's why you consider them bland.