Thank you so much to both of you.

Kevin Cruz - your insight that "class "product_tabs_ept" is not familiar to me" pushed me into right direction to look outside addons - I was an idiot and for hours I didn't cop on that my custom addons folder was outside addons folder. You should have heard me cursing that day! :D Based on that information I was able to find right place to edit and I had tab colour changed and set to red/white. I was not able to change actual tab background colour.

And here comes Jainesh Shah - thank you so much for providing that particular line of code. I could find "product_tabs_ept" in product.xml file, but changing bg-white to bg-black in there fixed nothing. Not sure if I had to reinstall whole theme maybe, but changing the same line in page source was doing the trick. I edited style_3.css in theme clarico and voila! success. 

for anyone looking for the same issue:

file to edit: your_odoo_custom_addons_folder/theme_clarico/static/src/scss/style_3.scss
file is pretty well described, so head to "07. Tab Section" and add new line and paste line given by Jainesh:
.product_tabs_ept .bg-white{background-color:#000 !important;}

next scroll a little bit lower to this section:
cursor:pointer;
a
{
    padding:0.6rem 1.5rem;
    (...)
    background: nth($f_color,18); -> add following line directly after this
    color: #ffffff; <---this will change text colour to white in active tab

just a little bit lower, in 
a.active
{
    (...)
background: nth($f_color, 19) <-- this is selected tab background colour. Just play around with number to find colour you like, i.e. 19 is primary theme colour. You can find list of all colours in the very first lines of this file: your_odoo_custom_addons_folder/theme_clarico/static/lib/theme/variables.scss

Thank you guys for the help and I hope this post may help someone in future with the same issue. Theme clarico support requested additional money for this "outside the theme" customisation and I'm strongly against milking customers like this as this was plainly theme customisation that should be included in EUR199+VAT price. Shame on them.

Unfortunately I'm not able to do more than just reply as i have no karma to do anything: I can not comment (minimum of 5 karma) nor I can mark best answer (minimum 5 karma). Have a good day everyone:)