Mastic or thinset for decorative glass tile mosaic?
Use mastic and a 1/8" v notch trowel. You can apply them directly to drywall since it is not a wet area. You'll also want to use a non-sanded grout. I've found that with some types of glass tiles the sanded grout will leave scratches and swirl marks on them. You should also still use grout sealer in case any water does splash up from the sink you don't want it to discolor your grout lines. Good luck!