Here I am going to explain you, how we can customize the look and feel of login page of a SharePoint online site.
Prerequisite to perform this action is , you should have global admin rights on the tenant.
As you know the default SharePoint online page looks like below (I am using personal free trail account, so the login page may look slight different from the licensed/organizational one).
Here are the steps to customize the login page
Login to your office365 admin page (https://admin.microsoft.com/AdminPortal) with your admin credentials.
Expand Navigation Menu (>) à Select Admin Centersà Select Azure Active Directory
Then it will redirect you to Azure portal (url looks like https://aad.portal.azure.com ), then select Azure Active Directory from left pane
Select Company Branding --> Select Configure --> You can see a page like below with all options.
Fill in the details and upload images as per your company branding requirements. Then you can see the login page like below : ( as i mentioned earlier, as this is a trail version you may see slight difference in the design but the approach is same)