WordPress MenuHeader Dissapears when adjusting opacity

I am using wordpress for a few weeks now and i have little coding experience but whenever i try to adjust the opacity it just fully dissapears instead of actually changing the opacity.

I am using wordpress on a localhost.

1. Menu still there (not fading) – https://gyazo.com/0cbbed0395308f85754b01f24a668672

2. Menu dissapeared. – https://gyazo.com/de0d1b815967078f44c9918feaa02c13

Please help me as i have no idea to fix this problem 😉