What beginner friendly RTS game can you recommend?

As a quick background, RTS is completely new to me. I spend majority of my time in FPS type games but want to branch out a bit and try out something different/new (to me).

Requirements:

• Somewhat casual (if that’s possible)
• Somewhat easy to learn
• Relatively modern with decent graphics (something that looks nice)
• Runs on Linux (of course) and Steam