First step

Hello,
I’m french gamer and président of gaming association. I have purshased you’r awesome theme but now i working slowly step by step for the best setting. My level is very low 😉

Questions :
It’s possible to input the owl carrousel and remplace the header picture in the homepage ?
I think for all translate i need to go in main.css but how can find it ? not in the wordpress admin menu ?
My website is online if you ok for follow my working and adjust my error.
Thank you for you’r great job sir.