Forum Discussion
SharePoint gear icon changes
Update (Apr 8, 2019): Please vote for this idea, if you face similar issue https://sharepoint.uservoice.com/forums/329214-sites-and-collaboration/suggestions/37322050-setting-custom-theme-for-new-su...
I think this link maybe help you.
https://techcommunity.microsoft.com/t5/SharePoint/SharePoint-Online-Settings-menu-change/td-p/357012