Is it possible?

Did you manage to widen the grey vertical strip? I looked at http://mido-subs.pl/ and the strip looks wider.

Just a heads-up that if you’ve made CSS changes in the origianl theme files, they’ll be overwritten the next time you update the theme. I’d suggest transferring the changes over somewhere they’ll be protected from future theme updates, either by using a custom CSS plugin (like the module in Jetpack or a standalone plugin) or a child theme.