Changing font size in Bonpress

Hullo!

I’m doing a small site for a charity and using Bonpress theme. I’ve paid for the upgrade and can easily see how to change the font size in the main body of text using the CSS box:

.entry-content {
font-size: 100%;
}

But I can’t work out how to change the font size in the left navigation bar. (the what we don/what we don’t do menu)

This is the site:

https://ernesthechtcharitablefoundation.org/

If anyone can help me I’d be thrilled. And relieved!

Thank you