Some questions about the header

Hi,
I’m setting up the tienda basic Version: 1.0.2 on a new website.
I’d like to know how does it works the header, since you give the recommended size of 1800 × 800 pixels, but even with those, the image doesn’t display as I want because it get cropped and zoomed (even if I don’t use crop).

Then the basic layout is really nice and clean, but it is too much white, and the contrast in the posts and in the menus is really low. Is there any way to change some colors (like the text) to make it a bit more readable?

Thank you, and great team, if it is working for my purpose I’ll definitely buy the pro version.