Thread:Project:Support desk/Modifying monobooktemplate.php/reply (6)

For editing the sidebar see Manual:Interface/Sidebar.

If for your navbar on the right you need an additional HTML container, then this might be possible using a hook. Another idea is to use the site notifice feature for this. See Manual:Interface/Sitenotice. CSS changes can be placed on the page MediaWiki:Monobook.css inside your wiki.

MediaWiki:Monobook.css also is the place to put CSS rules for putting an image into the search field.