Well I kinda use a mix of Bootstrap 4 and my own styles… mine is a business admin app, so lots of fields being read and written. My user base will be quite female dominated, so I have designed my own user interface styling which they can customise… so I use the BS4 containers, rows, columns etc but then use all my own styling on inputs, navs etc bringing in all the padding, margins, font sizes and colours. All my colours are variables as I have dark mode and will allow users to create their own colour scheme. I tend to hand edit my own CSS file so I can bring some order to its complexity, add comments that make sense, etc etc.
My scheme allows me to change the styling of hundreds of input elements with a few lines of edits in the CSS files and I need that control and flexibility!