Child Theme: impossible to modify a style

Hello,
I built a child theme starting from Nexus Theme (by Elegantthemes). I created a personalized style.css with all the modification I needed.

I also want to modify just a little configuration in a shortcode of “Schortcodes Ultimate”. I need to modify the title size of the [su_box] shortcode, managed from the box-shortcodes.css style sheet that is placed into the plugins directory, with the following path: plugins\shortcodes-ultimate\assets\css.

I am interested into changing the size of the title. So I found the .su-box-title style, I copied the entiere block with all the default info and I copied into my child’s theme css. The original box is the following:

.su-box-title {
display: block;
padding: 0.5em 1em;
font-weight: bold;
font-size: 1.1em;
}

I modified in that way in my child theme:

.su-box-title {
display: block;
padding: 0.5em 0.8em;
font-weight: bold;
font-size: 0.9em;
}

… I save my css, I upload it… but the change is not detected: I still see the original default settings from the original CSS.

After some try, I thought to write the same modified info into the THEME OPTIONS page, where there is a filed dedicated to Custom CSS.

But the info is still not detected.

How can I modify something from this plugin using the theme configurator or a child theme css?

I don’t want to modify directly the css of the plugin because in case of updates I will loose everything.

The website is: https://italicatv.it and the boxes are the 4 boxes in home page.

All the best,
Marco