Created attachment 3895 dark monitor with stand currently the "monitors" in the "set as wallpaper"-dialog are drawn in cairo, i would suggest to use pixmaps instead, mainly because it's easier to make it really pretty :) i attached a proposal of a dark monitor in vector-format i created that could be used for this purpose.
I agree that the dialog can look better. But it is really difficult to render a border around a rectangle with variable dimensions and proportions. The only way I can think of fixing that is the same way xfwm does the theming. Cutting the monitor up in smaller pieces, and letting them be filled with data read from pixbuf themes. +-+---------------+-+ |A| B |C| +-+---------------+-+ | | | | | | | | |D| |E| | | | | | | | | +-+---------------+-+ |F| G |H| +-+---------------+-+ | I | +---------------+ I think this is overkill for a preview widget. However, the color of the monitor-border should be retrieved from a theme-color. Because that is the only way it can look good on both dark and light themes.
The monitor is themable, it can be patched to make it look better but it wont be changed to load a pixbuf as example.