Is your feature request related to a problem? Please describe.
We have a Weblate instance at https://l10n.opensuse.org/ and its UI is based on Bootstrap 3.x. However, other openSUSE project websites are using a theme based on Bootstrap 4.x. If we deploy Bootstrap 4.x, there will be incompatible components/styles that may break the UI functionality.

Describe the solution you'd like
I hope Weblate can support Bootstrap 4.x.

Describe alternatives you've considered

Additional context