Are you using the starter templates from the Marketo Library or your own HTML code from your previous efforts and placing that into Marketo? Here's a link to the email documentation that might help out: Email Marketing - Marketo Docs - Product Docs
One issue that I've experienced with the default templates is that they were created/released before the "great shift" in 17q4 when google started reading @media queries. To the best of my knowledge, there hasn't been an internal effort there to re-create the templates. It might be best if you've got working code to add that in as your own template and use the Marketo Email Syntax to create editable areas (search: mktEditable) around the content you want to switch up.
To create a new template, you'll need a "container" and "mktEditable"-class elements for the 1.0 Editor, and "mktoText", "mktoImg", etc for Email 2.0 Email Template Syntax - Marketo Docs - Product Docs
You could check out knak.com for more modern templates, I believe they keep their templates updated so you may have more success on a quick-turnaround there.
Let me know if there's anything specific you've got questions about.