Step 1: Go to your WordPress dashboard.
Step 2 : Go to Appearance>>Customize
Step 3: Go to Addition CSS tab and open it.
Step 4: here you can add your extra css. You don't have to go with all file just to do basic changes on your website. Just put your CSS Here and press Publish.
and its done. Your changes updated successfully. Go to your site and refresh the page. you can see your changes on your site.