I'd like to know the class or ID of the theme menu I'm using.
Is there a way to find this value?
WordPress Development Stack Exchange is a question and answer site for WordPress developers and administrators. It only takes a minute to sign up.Sign up to join this community
I have just noticed you have solved your problem after I wrote this answer. I'm posting it anyway for future visitors.
Various web browsers have an "Inspect" function built in, for example; Firefox, Google Chrome, Chromium, Bravve etc.
How to Inspect pages
Load up the page you want to examine in an appropriate web browser and right click on the element you want to analyse and select "Inspect".
You will be presented with two screens, one with your original page and the other with information available through the Inspect option.
The "Elements" function (available at the top of the right-hand window) is the option you want. As you move your mouse over class identifiers and other code the related element on your web page will be highlighted.
Once you have identified the element you wish to inspect, click on the class identifier and the properties associated with it will be displayed. You can turn them off, on, modify them etc. Changes made here are temporary.
Other useful functions
The Inspection window is also useful for examining response headers from the server, current caching status, etc. It can be very useful for determining bottlenecks on your page.