How to create custom.css you can watch in this tutorial (it’s for Legenda theme but the process is the same).
2. You can add your code in Appearance > Widgets > Place in header top bar or edit the current links in the file wp-content/themes/royal/framework/shortcodes.php (lines 1729-1761) and change the links to yours, for example for twitter, replace the link https://twitter.com/share?url='.$permalink.'&text='.$post_title.' to your Twitter account link.
3. You can edit it in wp-admin > Settings > General.
