Is it the tools you want to learn or visual design skills?
The two are very different things.
Most tools for modern digital design can be learnt pretty quickly on the job but that doesn’t mean the designs you’ll produce will look professional.
If it is the visual design skills you want then it would be some sort of graphic design course that you’d need.
I started an online graphic design degree from a well-known organisation recently as despite doing this stuff for 20 years, I don’t actually have a degree in it but the course material ranged from irrelevant to out of date to just plain wrong so I complained, got my money back and am now writing my own graphic design course in a book which I plan to finish in a few weeks.
James Victore did an interesting talk which you can buy as a video course thing. It’s a good place to start.