Enigma color customization & mobile dimensions

Hello. I have found a lot of help on this forum. Thank you to all those knowledgeable in the area who contribute to help us.
Thanks to this forum, my site is almost customized. Although the menu hover works well, the color on the active page still needs to be revised, but im not sure how to access that . Also, how do I change the color of the dark blue area just above the footer ?

My final question is .. what are the optimal dimensions for the header that will not distort the image on mobile ?

Thank you in advance for all the help