The standard width of #site-branding is 20% so if you want to change it to 30% you will need to also change the widths of #site-description and #top-info
At that screen width the standard width of #top-info is 73% with a 7% right margin to leave room for the mobile menu hamburger icon.
(20% + 73% + 7% = 100%)
so to change #site-branding to 30% the #top-info (and #site-description) would need to be changed to 63%
(30% + 63% + 7% = 100%)