Child of 12045, because without fixing this : it make harder to make a feature for 12045

Child of https://bugs.limesurvey.org/view.php?id=12052 : no background for alert-danger in admin theme

I do not know if I understand you completely, but I strongly support the idea of using Bootstrap only in a plain vanilla way and overriding it in another .css file, therby making it possible to update bootstrap.

Possible structure:
bootstrap.css
ls-functional.css
ls-styling.css

And of course the distinction between functional and styling is not very clear, but we surely can find a way of working.

So: in core
bootstrap.css (plain)
ls-functional (only functional css statements)

and in template:
update-functional.css (for overrides of the functional stylesheet)
template.css (which is styling.css)
or a combination of these two...

And: yes, this is less efficient than putting it all in one file, but it is better for maintenance and for the end user.

@tammo : boostrap.css core are replaced by boostrap.css included in template if template want it.

The "issue" with boostrap.css are : it contains core +functionnality+theme+ etc ....

This is not the way LS do …

But :
https://gitlab.com/SondagesPro/SurveyThemes/skelvanilla
https://gitlab.com/SondagesPro/SurveyThemes/less-skelvanilla

less-skelvanilla use variables.less same than bootstrap.