Surefire way to get thicker, darker eyebrows.

Minoxidil.

>>12558893
There are some serums on the market but results vary. Other than that, brow mascara with fibers or get microblading done.

To grow thicker you can use any of these things before going to sleep: Latisse, Castor Oil, Olive Oil, Vaseline in before going to sleep.

I assume you are blonde or brunette
To dark them, you need to dye them but that's pretty gay, make sure nobody see you doing this stuff. Also if you are blonde, dye you hair to the same color as your eyebrows

>>12558897
DON'T DO THIS, YOU WILL LOOK LIKE SHIT This shit age you very fast and kill collagen

>>12558893
fue hair transplant

Tbh its not worth it. My eyebrows are ultra thick and its more annoying than anything else to have to constantly trim and pluck them to make sure they don't go wild.