Impossible to change background color

Hello, I installed the plugin and I really love it!

The only thing I have problems with is a simple customization. I cannot change the background color of the TOC block. I found a similar topic on this support forum and I inserted the suggested CSS code: https://www.ads-software.com/support/topic/changing-the-back-color-of-the-toc-section/

But It didn’t work for me.

Any other suggestion? Thanks