Rather than piling custom CSS rules on top of Bootstrap and fighting its defaults with increasingly specific selectors, the cleaner approach is to work with the Sass source files Bootstrap itself provides and change the variables they expose, such as colours, spacing scale, border radius and breakpoints, before compiling a custom build of the framework. This way customisation lives in one small, organised place instead of scattered override rules.

This approach also survives upgrades far better. Overriding compiled CSS directly means an update to Bootstrap can easily reorder or rename classes in a way that silently breaks those overrides. Changing the source variables instead means customisations flow through the same build process Bootstrap itself uses, so the framework and the design stay consistent as it evolves, with the added benefit of a smaller final CSS file since only the pieces actually used get compiled in.