change menus font color

Hi there, I’ve been digging for a few days and trying many ideas I found in the forums and none is working.
I would like to change the menus (topnav and header) font color to pure white. It’s not possible to do so via the Customizer so I understand I have to change the code in the main.css file. Since I am not a developer/coder I will have to use a plugin such as Simple Custom CSS. Would someone very kindly tell me what code lines I should write?

Currently the color is:
rgba(255, 255, 255, 0.7)

and I would like it to be:
rgba(255, 255, 255, 1)
or #fffffff (but I read that the rgba code takes over the # one…)

Thank you so much for any help!