What is the easy way to edit the maintenance / error 503 page?

Question

I'd like to modify the generic blank page with a custom template

Service Temporarily Unavailable

The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

Answer 1

Create/edit the errors/local.xml file like this:

<?xml version="1.0"?>
<config>
    <skin>customtheme</skin>
</config>

Then create a copy of errors/default folder and call it customtheme.

From this point, you can edit the phtml files as well as add the css files to create your own custom theme.

EDIT: even though it has been approved as a valid answer, this answer is only relevant for Magento 1 and I did not see it was a Magento 2 question.

Thus, the right way of designing the maintenance page is by editing the pub/errors/default/503.phtml page.

Answer 2

Actually, you don't need to edit core files for this purpose. Just copy pub/errors/local.xml.sample to pub/errors/local.xml, then make a copy of pub/errors/default (let's say pub/errors/custom). Change the skin value of the pub/errors/local.xml to the newly created errors skin:

<config>
    <skin>custom</skin>
    ....

After this you are able to make changes in pub/errors/custom/503.phtml and other files.