Hello Nikolay
If i may,
The code in the theme.scss file is not getting ignored by default. They are being applied in the site fine.
Problem is: The Live Demo version is only displaying what is rendered by the template only and from quickstart you are getting the components generated by the SP Page builder.
And the builder is doing fine rendering the template except for the tab element defined in "indigo\components\com_sppagebuilder\addons\tab\site.php"
Here on line 28 the css class (modern-tab) is wrongly prefixed by this string "sppb-" and making the css class as "sppb-modern-tab". Thus the rules defined in template css file for class "modern-tab" was not being applied.
You can replace that file with the following site.php file inside the zip file attached. I have removed the prefix from there, it should work fine now.
site.zip