Oi, Miguel,
I think what you want to do, exactly this way, is not possible in WP (I may be wrong).
What you can do, which would give a similar result, is:
1) Create a common page there in "Pages >> Add New", Take the header from it and everything else (those options at the bottom of the page editor), and insert everything in text into it, writing all your HTML there, and reference the CSS in "Custom CSS" (that the location depends on the theme). Now you go to "Settings >> Write/Read" (I don’t remember which one, but it’s the 3rd option) and you will select this new page as Home (static page). Ready, when someone accesses http://www.seudominium/ will fall on this page there, that you left no link, cebaçalho, menu or anything. Then to access the ADM panel just add the /wp-admin normally.
2) If (1) does not answer your question, I may have misunderstood and you want to fully personalize page 404. Go to "Appearance >> Editor" and search in a side menu to the right of the page (a list of files) for page 404. Then in the middle space, text editor, the contents of page 404 will appear. You will have a php code with "get_html" or "get_text" ... something like that. Take this out and check it out in HTML. Refer to CSS la in custom CSS.
In both cases, the CSS can be done directly in the style file (.css style or layout.css, depending on the language) in "Appearance >> editor".
In a third way of doing that...
3) You can still do so. Let’s say the client has the example.com domain, you use the WP there and create a directory inside the www (public_html) at the root of the domain, called "building" or anything that remembers that. Then install the WP in this directory there. Then the WP only accessed in "example.com/build/". At the very root, outside the directory, you create an index.html and an index.css just for this maintenance page.
I hope I’ve helped
Hug
Thank you very much, I think I’ll go with option 1
– Miguel