Menu font size will not increase.

Hi, I am fairly new and managed most things. I have tried to follow the CSS changes needed to increase the menu/sub menus font size to no avail. I copied and pasted the codes found on the internet with no change at all.
I have read and tried many self help solutions.
I did publish them. Is it because I have the basic package?
It is a village website with many oldies like me who need a bigger font to look at please?
Thank you.