How to reference a css class instead of completely importing the less file

[YuchiRO]

I'm using the roots theme for wordpress: https://github.com/retlehs/roots/It already comes with a precompiled bootstrap.css and recommends to use app.css for any customizations. As I don't have the options to add the classes to the buttons via html or javascript, I'd like to use the LESS sources to reference a css class, for example, I want to give a submit button the bootstrap button style:\[code\]input#submit{.btn;.btn-primary;}\[/code\]If I use \[code\]@import 'bootstrap.less';\[/code\] it adds the entire bootstrap css into app.css. How can I just use the bootstrap.less as reference for compiling?