How do I find my CSS file with our custom theme?

We don’t know how you can access files within your theme – Because it is custom built.

General:
Use a Custom CSS Manager plugin or Child Theme style.css file to hold your CSS modifications if you don’t want them erased if the theme updates.

click past the intro screen, and goto appearance – editor (usually in the left admin column)

https://codex.www.ads-software.com/Editing_Files

thanks for quick responses.
When I go to appearance/edit CSS I find this code:
/*
Welcome to Custom CSS!

CSS (Cascading Style Sheets) is a kind of code that tells the browser how
to render a web page. You may delete these comments and get started with
your customizations.

By default, your stylesheet will be loaded after the theme stylesheets,
which means that your rules can take precedence and override the theme CSS
rules. Just write here what you want to change, you don’t need to copy all
your theme’s stylesheet content.
*/
I understand I can type new code to be entered to the CSS. I want to SEE the CSS Stylesheet and delete an image we have been using. From my limited ability to understand, I think I will need to go to the ftp of the server then open the stylesheet to make changes. If this is so, then what use is the option in WP? It seems that the newest WP has made this feature is a little over-automated.
Again, thanks for the quick replies.

instead of ‘appearance – edit CSS’, goto ‘appearance – editor’ where you might be able to see and edit the theme’s stylesheet

thanks, but I am taken to the same page by either link.
I am right now cruising through theme files on the server/ftp.
Thanks to all. I will find it the hard way. I still think WP has over automated the editor functions.
Cheers!