I want to customize the search box look and feel, I read an article about doing it using a delegate control, but that works only on prem, how do I do that in O365, I want to custom the textbox and make a custom button with some text on it like Search.
You want to change the Search Box Control Template. Look in the Master Page Gallery/Display Templates/Search for the Control_SearchBox.html file. (Depending on the site template you may not have the HTML files. You may have to enable the Publishing feature.) Alternatively you can copy the HTML from another instance of SharePoint and copy it to this folder.