three columns…is possible ?

Do not try modifying the width on skeleton.css.
Instead go to the loop.php file on your theme directory and change this line

<article id="post-<?php the_ID(); ?>" class="post columns four <?php echo sampression_cat_slug(); ?> <?php if ( is_sticky() && is_home() ) { echo 'sticky corner-stamp'; } else { echo 'item'; } ?> ">

to

<article id="post-<?php the_ID(); ?>" class="post column one-third <?php echo sampression_cat_slug(); ?> <?php if ( is_sticky() && is_home() ) { echo 'sticky corner-stamp'; } else { echo 'item'; } ?> ">

Hope this helps.

What’s the point of modifying the theme’s files like that when those modifications will just be erased when the theme updates?

Hi Andrew Nevins,
If you are worried about that just make a child theme and make modifications. Check this link to know about child themes on wordpress.

@dharmapoudel – LOL – he certainly knows about child themes – the point is that you were advising someone to modify theme files without using a Child Theme. That’s not really helpful. If you want to help out that’s great – but help the OP to make changes in a way that won’t be lost when the theme is updated – explain how to use a Child Theme.

two columns is it possible?

<article id="post-<?php the_ID(); ?>" class="post columns one-second <?php echo sampression_cat_slug(); ?> <?php if ( is_sticky() && is_home() ) { echo 'sticky corner-stamp'; } else { echo 'item'; } ?> ">

does not work

I was able to transform into two columns, changing skeleton.css. The problem now is that the images on the home are blurred.

.container .four.columns { width: 460px; }

I was able to transform into two columns, changing

<article id="post-<?php the_ID(); ?>" class="post columns eight <?php echo sampression_cat_slug(); ?> <?php if ( is_sticky() && is_home() ) { echo 'sticky corner-stamp'; } else { echo 'item'; } ?> ">

but the photos are always blurry. 700×400 px