In Keynote you can create custom themes that show up in the Keynote theme chooser. Apple's (newer) themes support all 16:9 resolutions (like 1920x1080) but also all 4:3 resolutions (like 1024x768).
if I create a custom theme at a resolution of 1920x1080 and save it, the theme chooser will only give me options of resolutions with a 16:9 aspect ratio, no 4:3 resolutions.
So there's a 1920x1080 option and a 1280x720 option BUT there's no option for 1024x768 or 800x600.
Same goes the other way around.
How do I create a custom theme that supports ALL resolutions/aspect ratios, like Apple's own themes?