Stylesheets compress well. They are repetitive by nature, full of indentation, and usually written with generous spacing to keep rules readable. Removing all of that typically cuts a hand-written stylesheet by 20% to 30%.
That matters because CSS is render-blocking. A browser will not paint anything until it has the stylesheet, so every kilobyte sits directly in front of your first contentful paint. HTML and JavaScript can be deferred. CSS generally cannot.
What the minifier does
Comments are removed. As with HTML, these often say more than intended - the name of the theme you started from, a note about a client, a block of rules commented out during a redesign and never deleted.
Whitespace is collapsed everywhere it is not needed. The spaces around braces, colons, semicolons and commas all go, along with the indentation and the blank lines between rules. This accounts for the bulk of the saving.
Hex colours are shortened where the shorthand is exact. #ffffff becomes #fff, #aabbcc becomes #abc. Only colours where each pair repeats can be shortened, so #1e93e8 stays as it is. Three bytes saved per colour is small, but a stylesheet with a design system in it can contain hundreds of colour declarations.
Zero values lose their units. margin: 0px becomes margin: 0, which is equivalent in every browser - zero is zero regardless of unit.
The final semicolon in each rule block is optional in CSS and can be dropped. The option is off by default because some older build tools and CSS-in-JS parsers are unhappy with the result, and the saving is a byte per rule.
What it will not do
This minifier does not restructure your CSS. It will not merge two rules with identical selectors, remove properties overridden later in the file, or delete rules whose selectors match nothing on your site.
That restraint is deliberate. Those transformations require understanding cascade and specificity, and getting them wrong changes how the page looks. A minifier that only removes characters is one you can run without re-testing every page. If you want the aggressive optimisations, use a dedicated build tool and budget time for regression testing.
Order still matters
One thing to keep in mind: CSS depends on source order. When two rules have equal specificity, the later one wins. Minification preserves that order exactly, so behaviour is unchanged. But if you are concatenating several stylesheets before minifying, concatenate them in the same order the browser would have loaded them - reordering files is how a minification step ends up blamed for a visual change it did not cause.