Button styles not being overridden by custom.css

Where are the background-image and background-color css settings for buttons being controlled in EasySocial? The color is set in the configuration, but even when I have rules in my custom.css to override the background color and image, they are not being applied. You can see in my screenshot they are being overridden by the default settings. How can I make my custom.css rules apply? The default styles don't match my design.

I put my styles in both of these locations and neither one will override the defaults:
public_html/templates/rt_xenon/html/css/custom.css
public_html/templates/rt_xenon/custom/scss/custom.scss