Generally, when you decide to introduce a certain framework, you need to focus on the style sheet of this framework and make some detailed adjustments on this basis. In other words, your style.css may need to be changed.

Of course, you can also consider changing the style in the bootstrap source file, but it is not recommended to change the structure. It is still possible to only change some color matching, border effects, etc. Changing the structure may cause layout confusion.

If you need to use two style sheets in different places on a page, you can consider adding restrictions by region. Of course, in this case, it is best to change your own style.css and put it in the bootstrap style sheet. Import it later, such as

Original

a {}
.abcd {}

Add a restriction (you also need to add a p.mime in the HTML to wrap it up (I wrote LESS to make it easier to see the hierarchical relationship)

// less file
.mine {
    a {}
    .abce {}
}