A site built on the default CDN stylesheet looks like every other Bootstrap 2,007 site and ships about 232 KB of CSS, most of it for components the page never uses. Bootstrap gives you four levers: Sass 22,539 variables and maps change the design at compile time; CSS variables and color modes change it at run time without a build; the utility API adds, modifies or removes utility classes; and lean imports plus PurgeCSS 955,901 strip what you do not use. Under the Hood explains the machinery; this section applies it to one real Vite 25,978 project.