Did you add your custom.css like this:
Add:
{% do assets.addCss(‘theme://css/custom.css’, 98) %}
To (depending on your theme name):
templates/partials/base.html.twig
You might have to add it to head or header.html.twig
As seen here under step 5:
add it after style.css.
For more on overriding plugin css: Override plugin css