When you creating a new Master Page/Theme/CSS for your SharePoint site and you want to apply them across your site including your sub-sites . This can be easily done from Site Settings --> Look & Feel --> Master Page.
If the 'Master Page' option is missing under the 'Look & Feel' section ,there is a simple way to re-enable it by activating certain Site features.
These are the simple steps.
Step 1 : Go to Site Settings -> Site Collection Features .
Step 2 : Click the Activate button for the following features shown in the below figure
Step 3 : Go to Site Settings -> Manage Site Features .
Step 4 : Click the Activate button for the following features shown in the below figure
Step 5 :Now go to to Site Settings -> Look & Feel and you will be able to see the 'Master Page'.