Adding your own style sheet works when you are trying to do something quick or when the modifications are minimal. Customizing Bootstrap beyond small changes involves using the uncompiled Bootstrap source code. The Bootstrap CSS source code is written in LESS with variables and mixins to allow easy customization.
LESS is an open source CSS preprocessor with cool features used to speed up your development time. LESS allows you to engage an efficient and modular style of working making it easier to maintain your CSS styling in your projects.
The advantages of using variables in LESS are profound. You can reuse the same code many times thereby following the write once, use anywhere paradigm. Variables can be globally declared, which allows you to specify certain values in a single place. This needs to be updated only once if changes are required.