Themes, what are they?

[CamillaT]

I do not understand this with the themes. What is it I decide when I choose a theme? I understand that it is not only clean appearance, but it is more behind. How do I know?
I chose the theme 'Dusk' because I wanted the navigation bar at the bottom. I want that my first page to be an image with a text beneath and there below the navigation bar. I have managed to make such a page that looks ok when I edit. But when I publish it, and is looking at it 'in real' on my desktop the page will be too large; I have to scroll to see the whole, both in height and width. Why? I want the navigation bar at the bottom of the screen, and I would prefer to be able to scroll if needed and still have the navigation bar there all the time.
Furthermore, I expect to see my whole image, centered and adapted in size, both on the computer screen and mobile.
How do?
Also: It must surely not have a background image? How do I say that I just want to have a white (or other monochrome) background?

[Queso]

A theme is, essentially, the "basics" of how your site displays. The navigation bar, the footer and header details, and other underlying essentials are controlled by the theme.

You shouldn't generally need to scroll horizontally on your site, unless your screen is fairly small. We haven't encountered it on your site so far, either. What pages are you seeing that effect on?