df74f0734a
Currently in a theme the background files are named `background_<tab name>.png`. The problem is that sometimes retro-go adds new tabs, which causes themes to have missing backgrounds. So I added a fallback. Theme can now provide a file named `background.png` that will be used when a tab-specific background isn't found. This also enables themes to provide a single background file if they don't want per-tab ones.