bartik subtheme and css

I want to make some modifications to the bartik theme i am using D8 (like changing the logo position,etc) i created a subtheme named it “mytheme” following the instruction i found here:

https://www.drupal.org/docs/8/theming-drupal-8/creating-a-drupal-8-sub-theme-or-sub-theme-of-sub-theme

So, first I created the mytheme.info.yml file containing:

name: mytheme type: theme base theme: bartik description: subtheme for bartik core: 8.x libraries:   mytheme/global-styling regions:   header: Header   primary_menu: 'Primary menu'   secondary_menu: 'Secondary menu'   page_top: 'Page top'   page_bottom: 'Page bottom'   highlighted: Highlighted   featured_top: 'Featured top'   breadcrumb: Breadcrumb   content: Content   sidebar_first: 'Sidebar first'   sidebar_second: 'Sidebar second'   featured_bottom_first: 'Featured bottom first'   featured_bottom_second: 'Featured bottom second'   featured_bottom_third: 'Featured bottom third'   footer_first: 'Footer first'   footer_second: 'Footer second'   footer_third: 'Footer third'   footer_fourth: 'Footer fourth'   footer_fifth: 'Footer fifth' 

Next i created the mytheme.libraries.yml containing the following:

global-styling:   css:     theme:       css/style.css: {} 

i went to appearance and enabled “mytheme” as default, went back to site, looks identical to how bartik was so far so good

my problem now is, how do i move on with the customizations i want to do?

i created a folder called: “css” in “mytheme” directory, inside the css folder i created a blank file called “style.css” and there i wrote the first modification i want to do

.region-header .site-branding {     float: none;     text-align: center; } 

When i go back to site i see no difference, the logo remains on the left like it was on bartik

Can somebody help please?

I am not sure how to proceed and if what I did by creating a css folder, etc is the right way.

Thank you.