HubSpot does not use Bootstrap as-is for their CMS and grid system. Instead, its default 12 column grid system and class naming is based on the Bootstrap 2 grid system, but the actual code is different. Here is what the default HubSpot layout.css file looks like.
HubSpot 2 column grid (Bootstrap 2) div structure example
<div class="row-fluid ">
Column 1 content
Column 2 content
So can I use Bootstrap 4 or another CSS framework like Tailwind CSS with HubSpot?
Yes, you can. The only caveat is any default modules or functionality coming from HubSpot will be written to support their internal grid system and not your custom one. So you will have to keep both. This isn’t hard to do but just something to be aware of. I have found it easier to follow HubSpot’s lead in their grid system naming and structure but then overwrite the actual code to work differently with flexbox. This way, if a client ever contacts HubSpot support, they will be able to make adjustments without having to relearn a new CSS framework. Simple systems and processes make future maintenance a lot easier.
But if you are comfortable with your build system and CSS framework, it is manageable. Here is a theme framework that is built using Bootstrap 4.