Why dropdown isn't working in dash_table.DataTable and best ways to solve it

Hi everyone, this is my first contribution to this forum - I’m looking forward to becoming a part of your community!

I have a problem with dashtable-dropdowns not working:

The issue can reportedly be solved by adding css: .Select-menu-outer { display : block !important; } - but I can’t find exactly how to add the css -
I tried putting it in a file ‘assets/dashtable.css’ but it didn’t help.

Can anyone help?