I recently tried using some glass paint. I used PermaGlaze brand that doesn't need to be baked. I found it kind of challenging to work with because it was thin. I'd be afraid to use a stencil with it because it would probably run under it. It even ran under the masking tape I used on one experiment.
Maybe you could transfer the design in wax pencil or something that would resist the glaze and then fill it in. You'd definitely want to work with it flat.
I think deep red or deep purple are the most romantic colors for a bedroom. I wouldn't do the whole room in either color, but perhaps look for a color scheme that uses one of those colors for the walls or the bedding.