Stylesheet not working in WordPress

I just installed a new theme, and am having trouble

Which theme? where did you download it from?

https://codex.www.ads-software.com/Forum_Welcome#Include_as_Much_Information_as_Possible

It was a custom made theme.

– you’ll be more likely to find a good answer from your theme’s developers, for they have the access to its code. https://codex.www.ads-software.com/Forum_Welcome#Commercial_Products

here’s the website in question: https://3da.2c7.myftpupload.com/
BTW, I know how to make edits in the css file and so forth, BUT when I do the changes DON’T take effect. NOTHING happens.
I’ve tried EVERYTHING I can but to no avail. I just wanna change the header background and slideshow back, that’s it! PLEASE HELP!

BTW, I know how to make edits in the css file and so forth, BUT when I do the changes DON’T take effect. NOTHING happens.

Where are you making CSS edits? Can you give a specific example that we can check?

I went to Appearance>Theme editor>style.css and changed the background color to #ffffff,

Okay, that’s not where you make CSS modifications.
Use a plugin: https://www.ads-software.com/plugins/custom-css-manager-plugin/