customize colophon row fluid

Yes it is.

What’s the end result you’re looking for, in relation to the other 2/3 of the bottom of the footer? If you increase the centre span4, what do you want the other span4s — the social links and the “back to top” — to do?

social span2, credit span8 and back-to-top span2…

If you create a child theme, every time WP needs a theme file, it looks for it in the child theme. If it’s not there, it loads it from parent.
So all you need is copy the file you want to modify in your child theme (keeping the same folder structure) and mod it.
If the file you have modified gets upgraded in future versions of the theme, WP will still load your child theme version, so you will basically lose all upgrades applied to that particular file.
But you can always take the upgraded file from parent, reapply your mods and replace the one in your child theme.

Alternatively, a more future-proof way is to use the following in your functions.php:

add_filter( 'tc_colophon_left_block_class', 'my_colophon_left_block_class' );
function my_colophon_left_block_class() {
    return 'span2 social-block pull-left';
}

add_filter( 'tc_colophon_center_block_class', 'my_colophon_center_block_class' );
function my_colophon_center_block_class() {
    return 'span8 credits';
}

add_filter( 'tc_colophon_right_block_class', 'my_colophon_right_block_class' );
function my_colophon_right_block_class() {
    return 'span2 backtop';
}

If the middle section doesn’t resize correctly, check you aren’t also modifying the span in any code that you may have already implemented to change the credits.

Perfect!! Thank you very much ElectricFeet ??
One question: where can i find a list of all the class used by this theme?
Just to learn something… thanks again.

There isn’t one, but here’s a good summary of some of the main ones: https://www.themesandco.com/customizr/css-glossary-for-the-customizr-wordpress-theme/

ok, thank you very much.

@electricfeet,
Thanks!!! Exactly the code I was looking for!!!