Minifying or condensing CSS code can help browsers avoid using too much data during functions such as downloading or execution time.
Ways to minify
Getting rid of spaces, indentation, comments, etc., in your CSS file can reduce the amount of bytes it takes to open a stylesheet on a site. Although it may make your code look cleaner, they are unnecessary for the code to load faster.
Also combinining multiple CSS files into one can help load time as well.