Cascading Style Sheets (CSS)

Capitalize references to the technique for adding fonts, colors, and so on to web content. Spell out unless the abbreviation CSS is familiar to your audience.

Lowercase references to style sheets created using the technique. Don't use CSS to refer to a specific cascading style sheet. Instead, use the CSS file, the cascading style sheet, or the style sheet.

Examples
If this is your first time working with Cascading Style Sheets (CSS), don’t worry.
Use a custom style sheet to create a unique look for your SharePoint site.
It looks like there's a problem with the CSS file.