H5P Mods Wordpress Plugin Master - color changing of button does not work

Hi to all,

I am running H5P on a WordPress-Website. I want to change the color of the button "Check" from blue into another color.

So I have installed the plugin "h5pmods-wordpress-plugin-master.zip" from https://github.com/h5p/h5pmods-wordpress-plugin.

I changed the "custom-h5p.css" like said in the documentation and uploaded the files into the directories.

But: Nothing happens. All buttons are still blue.

I am not very experienced with PHP or CSS. I only wanted to put the new color into the file where it belongs. But I guess the problem is another.

Is someone out there who can help, please?

Thank you very much in advance,
Konstanze