Se atente a ordem de prioridade do seu CSS. Geralmente trabalho com o arquivo bruto do bootstrap e um CSS adicional. Com isso, mantenho o bootstrap na forma original e vou realizando as alterações em uma outra folha de estilo a qual, importo posteriormente a do bootstrap. Resumindo, meu header ficar mais ou menos assim:
A Computer Science portal for geeks. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. Bootstrap (Flexbox) is still better than CSS Grid for creating Layouts. RE: Per Harald Borgen's Bootstrap vs CSS Grid ... Just add a few lines of CSS… /* get the main container to fill height ...
Bootstrap. Build responsive, mobile-first projects on the web with the world's most popular front-end component library. Bootstrap is an open source toolkit for developing with HTML, CSS, and JS.