Add Template CSS

I've just updated to the latest EasySocial/Easyblog combo and am trying to change the colour of the Search button but can't work out how to do this.

There's a new stylesheet that came with the new Easyblog update, style-modules-5.1.8.min.css

It contains the following style:

#eb .mod-btn-primary {
color: #fff !important;
background-color: #1e88e5 !important;
border-color: #1e88e5 !important;
}

I tried to override this using custom.css but this didn't work.

I also tried adding a template override css, ie. style-modules-5.1.8.min.css, to the folder:
/home/echoacti/public_html/templates/ja_obelisk/html/com_easysocial/styles
but this didn't work.

Is there another way to override this?

Cheers, David