Hide Change the Look
You may not want your users to have easy access to change the look and feel of the SharePoint site they have access to. You should always ensure that users have the exact permissions they need– and nothing more, but in any case you can hide the option to change the look of the site from the site actions/gear menu.
Want to hide it?
Drop this code in:
.ms-siteactions-root .ms-core-menu-list li[onmenuclick*=”designgallery.aspx”]