Apply top level site UI to all the Sub sites

1. Create the Top Level Sites as per the requirement

2. Change the UI as per the requirement

3. Save the Changed UI as Custom Template

Site Action --> Site Settings --> Save “Site as Template”

A) Enter the FileName: CustomTemplate (Change the file name as per the requirement)
B) Template Name: CustomTemplate (Change the template name as per the requirement)
C) Template Description: CustomTemplate (Change the Description as per the requirement)
D) Finally click ok

4. Active on "Office SharePoint Server Publishing Infrastructure" features.

Site Action --> Site Settings --> Site Collection Administration --> Site Collection Features --> Activate "Office SharePoint Server Publishing Infrastructure" features

5. Activate "Office SharePoint Server Publishing" features

Site Action --> Site Settings --> Site Administration --> Site Features --> Activate "Office SharePoint Server Publishing" features

6. Create the Sub site and inherit the UI from Top Level Site

Site Action --> Create or Create Site

A) Title: As per the requirement
B) Description: As per the requirement
C) URL name: As per the requirement
D) Select a Template: Click the Custom Tab --> Must Select the "CustomTemplate".
E) User Permission: As per the requirement
F) Finally click "Create" option.

7. Apply Top Level site header to sub site

Go to Top Level Site --> Site Action --> Modify All Site Settings --> Look and feel --> Master Page

A) Site Master Page --> Select Default.master --> Check "Reset all sub sites to inherit this Site Master Page setting" check box

B) System Master Page --> Select Default.master --> Check "Reset all sub sites to inherit this system master page setting" check box

C) Finally click ok