Change Nav Bar Letter Spacing (‘Additional CSS’ Not Doing Anything?)

I’m trying to get rid of the letter spacing in my nav bar list items on my site.

From the dashboard I went to appearance > customize, and within the editor I was able to change some text stylings within header > nav style. There is limited control here because I can only change what there is a slider for, and there isn’t one for letter spacing. In chrome I right click, inspect what is styling it and it’s showing that it’s “.fl-row-content-wrap”. I confirm this is indeed what’s affecting it by changing the letter-spacing command within inspector from 0.2em to 0em and it has the desired effect.

The only css sheet for the site available to edit I can find is within the same ‘customize’ menu under ‘additional CSS’, but nothing I put in there seems to have any effect on anything at all (I’ve tried in the past with no success). Anyways, I still try to put it in:

.fl-row-content-wrap {
letter-spacing: 0em;
}

…and to no surprise, it doesn’t work. I assume something is overriding it but I don’t know where/what it is and how to fix it because I don’t know of any other places to customize css from within the wordpress dashboard. I’ve read something about clearing caches being a fix for the ‘Additional CSS’ not doing anything, but I don’t know what a cache is, where to locate it, or how to clear it.

Also, I’m not sure if this is relevant to my problem or not: I might be using a theme? I didn’t start the site and I don’t know how to tell.

The page I need help with: [log in to see the link]