After installing Bikeshed you can start making changes to its source code and contribute those changes back via pull requests. (There is no need to install it again after having made changes.)
bikeshed test will run all tests which may take a while, for development bikeshed test --manual-only is preferred. You can also run bikeshed test test.bs to run an individual test, where test.bs is a resource in the tests/ directory. To change the expected outcome of a test, you can run bikeshed test test.bs --rebase.