The issue is that the feature is not enabled that allows the layouts.

To fix the issue go to Site Actions -> Site Settings in the upper right corner.

Under "Site Collection Administration" click on "Site Collection Features".

Look for "SharePoint Server Publishing Infrastructure" and activate it. It might take a moment to load.

Next return to "Site Settings" and click on "Manage Site Features"

Look for "SharePoint Server Publishing" and activate it. It might take a moment to load.

OOB Feature 

1. Enable "Document ID Service" in the Site Collection Features
2. New entry in the Site Collection Administration appears: "Document ID settings"
3. In this menu you can define a prefix for your Document IDs
4. In any list/library a new column is provided: Document ID (linked to document)
5. Display this column in a list view --> use the provided URL of this column for linkage

http://test/_layouts/15/DocIdRedir.aspx?ID=DocIDPrefix-SomeNumbers-ID

http://test/_layouts/15/DocIdRedir.aspx?ID=DocIDPrefix-12231-987

 CSS, Selectors and its rules for CSS Specificity and you will be able to master any CSS problem.

In your scenario you can wrap your content in a DIV unique  selectors ID

<div id='myCEWP'>
  <H1>Hello World!</H1>
</div>
And then apply your CSS only to that one H1 in the DIV :

<style>
   #myCEWP > H1 {
     color:green;
   }
</style>

SP.SOD is a nice feature of SharePoint.

function message()
{
alert("Hello World")
}
ExecuteOrDelayUntilScriptLoaded(message, "core.js");