Why is the plural acronym "CSS" treated as singular?

CSS stands for Cascading Style Sheets, which is a plural phrase. One would say:

Cascading Style Sheets are used to provide a central location for all of the stylings of a project.

However, it seems to be the prevalent usage to say:

CSS is used to provide a central location for all of the stylings of a project.

Why do we do this, and is this correct? Would anyone ever say "CSS are used to provide a central location for all of the stylings of a project"?

CSS here has evolved as a noun independent as to what it means as an acronym. It refers not just to "cascading style sheets" themselves, but to the entire system and infrastructure for styling web pages.

In short, it is a singular noun that has come into existence due to common use.

Here is a similar example: Imagine a company called "Unlimited Designs." judging by the name, you might say something like "Unlimited Designs is going out of business." It's a singular noun, even though designs is plural.

First, according to the specification, “Cascading Style Sheets” or “CSS” is the name of a specific language, and therefore there is nothing wrong with treating them as singular.

Second, people sometimes refer to a style sheet written in CSS as “cascading style sheet” (for an obvious reason), although this usage does not appear in the spec if I am not mistaken. In your example

Cascading Style Sheets are used to provide a central location for all of the stylings of a project

“Cascading Style Sheets” is used in this meaning, and therefore it is plural.

In general popular acronyms, with time, tend to be used as independent words without always thinking to their original expanded form. This doesn't only result in mismatched singular/plural forms, but also in other humorous inconsistencies, such as the RAS Syndrome (a.k.a. Redundant Acronym Syndrome Syndrome).