WordPress own theme development from 0 and 3rd party plugin .css

Colleagues!

I am making own tourism website using WordPress. I have own design, built from 0.

Main page layout works perfectly and represents mine design wishes idea.

It is shown below:

http://unclealextravel.online/?page_id=98

For selling tours, I use 3rd party plugin called “WP travel”. It works perfectly with general themes, developed by others. But when I switch to mine own theme, there is a problem. Mine own theme does not include correct .css, which belongs to this 3rd party plugin.

http://unclealextravel.online/?ttbm_tour=the-mentalist-tickets-las-vegas

What should I add to functions.php to fix the issue with these 3rd party .css?

Thanks in advance!

BR,

Vadim Turchinovich

Tried to add different combinations of codes to functions.php