I feel like this should be obvious to find but I have not come up with anything. Bootstrap 4 has utility class for adding and removing a border, but is there support for def
If you have bootstrap npm library and if you look at _variable.scss you will find $border-width: 1px !default;
You can override it by creating a new scss file where you can import bootstrap modules.
$myborder-width : 3px;
$border-width:$myborder-width;
@import "../node_modules/bootstrap/scss/bootstrap";(you can include only the required modules here)
you can convert this main.scss file to css and use this css instead of bootstrap.css in your html.