In SCA it is suggested not to use direct values for different screen sizes
Hence we need to use screen sizes as variables accordingly
To get variable in themes navigation is given below to find variables
theme/Modules/twitter-bootstrap-sass/3.4.1/assets/stylesheets/bootstrap/_variables.scss
For example 768px we can use $screen-sm-min
Screen sizes here will be defined for different devices such as tablets, mobile, desktop, laptops so we should make use of these variables while calling them for responsivness
Like wise not only screen sizes, some default paddings margings colors will also be available here in this files