How do I change the Font size of a Sub Menu

Okay, getting kind of frustrated so I hope you can help with this issue.

I really want to change the font size of the sub menu items on my menu bars. They are just really small and hard to see.

I reached out to the general WP community at the WordPress.org support forums and saw several posts with a similar request. Several users suggested different snippets of CSS, but none of them worked for me. Here are some examples of the ones I loaded into Custom CSS.

.sf-menu ul li a {
font-size: 1.2em !important;
}

#main-navigation .sub-menu li {
font-size: 1.3em !important;
}

#top-navigation .sub-menu li {
font-size: 1.3em !important;
}

#nav-second ul.sf-menu ul li a {
font-size: 12px;
}

#submenu a {
color: #FFFFFF;
font-size: 150%;
font-weight: bold;
padding: 3px 5px 3px 7px;
text-shadow: 0 1px 0 #555555;
}

I then posted a request of my own to the WP support forum and a guy named Andrew responded with the following:

Andrew
Forum moderator
Posted 17 hours ago #

This works for me:

.sf-menu li {
font-size: 1px;
}

If that doesn't work for you then try adding this:

body {
border: 10px solid red;
}

I tried the first bit, and it did not work either.

Then I added the second code with the solid red border. That worked, but I don't want a solid red boarder, so I removed the code, and now I can't get rid of the border!!!!

I responded to him that it did not work, so he said the following:

Andrew
Forum moderator
Posted 17 hours ago #

You may have a CSS syntax error, if the body 10px border style didnt work

Try this https://jigsaw.w3.org/css-validator/

I went to the CSS-validator plugged in my URL and it says I have 78 errors and 835 warnings.

I don't think I've done anything crazy with my site, so I'm really surprised by the results of this CSS validator.

Can you guide me on how to modify the font size, and whether I need to do anything abot 78 errors and 835 warnings?

Thank You