style.css sheet

I cannot figure out why adding lines of css to the style.css sheet in the wordpress editor is not resulting in any changes to the site. Where is the primary css stored for the site? I can’t find any of the sheets that I can edit.

My website is https://www.compassionatehouston.org and I would like to add a text shadow to the description text in the slider, for example. Where can I access the css for that?