I feel like basic html and css can get you pretty far. You don't get stuck on html and css issues the same way you do with javascript. A quick google search will almost always help you figure out exactly what you want.
But having said that, I would still spend more time learning them. If you've invested the time to master React js and javascript, html and css will come easy. It's like being a great storyteller but not wanting to write a book because you're slow at typing. I find git way harder than html and css but it's basically something you have to learn if you're serious.