CSS Modification for mobile devices

Hello,

I want in .su-expand_content (CSS: box-shortcodes.css) a different font-size for mobile screens than in desktop (default) view. I`ve tried to add the following code on top of the box-shortcodes.css, but without any effect:

This is an example for iPhone5 resolution:

@media only screen and (min-device-width: 320px) and (max-device-width: 568px){
.su-expand-content{
font-size:14x;
}
}

My thoughts:
– is my code completely wrong?
– do I have to put something in PHP as well?
– Maybe I have to add it somewhere else?
– Do I have to take “.su-expand” class into the @media as well?

BR
Sebastian

https://ww.wp.xz.cn/plugins/shortcodes-ultimate/