how to set the child themes?

I’m a newbie with child themes and want to know how can i get the same look of the CityNews with its mailing list style and everything else ..
https://dp.wlthemes.com/?skin=basic_CityNews
when i uploaded the theme to my website and mark it as supposed to be and choose it it dos not apply the css, also all child themes that i uploaded still giving me the same template (gray!) .. please help me with this theme setting this is my link:
https://uaewhoswho.com/