[Theme: MesoColumn] Fixed width for desktop and flexible for mobile

hello,

i check the faq.txt and can now make the site fixed width.

i was learning about responsive themes
and figure out:

i want the site for desktop on fixed 900 pixels and for mobile devices be flexible?

is that actually possible or im asking nonsense?

my concerns are, for mobile its fine, but for desktop, my screen is 1920 pixels wide, so its just looks a bit too wide. I notice the theme doesnt extend up to 1920 pixels wide, its about 1400 pixels max,

is there a way to edit this “1400 pixels” maximum width to make it 900 pixel?

in other words, i want it fixed in 900 for people having a higher resolution than 900 in width, as in laptop users.

and i want it flexible for people having a lower resolution as in phone users with a 320 pixels width.

also is it normal for the theme to have the sidebar at the bottom on an android phone? i open the site on a phone, and didnt edit any css.

thanks for listening for this long post.