Do the following to maintain a common, consistent look and feel throughout the top-level site and all subsites.
From the Top-Level Site, navigate to the following settings section:
Site Action -> Site Settings -> Look and Feel -> Master page
Site/System Master Pages & Themes: Select radio button "Specify a master page to be used by this site and all sites that inherit from it:" and select desired design. Also, check the box next to "Reset all subsites to inherit this site master page setting".
Do the following to enable tree view on the left navigation area in the SharePoint Online Team Site.
From the Top-Level or Subsite, navigate to the following settings section:
Site Action -> Site Settings -> Look and Feel -> Navigation Elements
Enable Tree View - Specify whether a tree view should be displayed to aid navigation. The tree view displays site content in a physical manner.
Do the following to change the top navigation links after subsites have been created to hide/unhide them.
From the Subsite, navigate to the following settings section:
Site Action -> Site Settings -> Look and Feel -> Navigation
Global Navigation: Select "Display the same navigation items as the parent site" if you want to see all the other subsite links and be visible on there as well. Also, check "Show Subsites" to have subsites grouped under the main links.
Reference Links: https://support.office.com/en-us/article/Configure-the-top-link-bar-for-site-navigation-1B614211-2882-4507-A791-8716090C9905?ui=en-US&rs=en-US&ad=US