Why would changing a single bg color default in the Marketo Variable Definitions (source code view), saving it and doing *nothing else,* make the drag/drop modules become permanently unavailable in a Marketo template?!
FROM <meta mktomodulescope="true" class="mktoColor" id="body-sec7-bg" mktoname="Body-Sec7 BG color" default="#e0e0e0" />
TO <meta mktomodulescope="true" class="mktoColor" id="body-sec7-bg" mktoname="Body-Sec7 BG color" default="#fafafa" />
We would really need to see the whole set of code to be able to tell whether there's another code issue at play here, but I have seen the loss of module functionality happen sometimes from a saving error. As a first step you can always re-save the template and see if that fixes it, or copy the code into a new template and see if it works in an email based on that iteration. If either of those solutions work this was likely just a bug. If neither work there's likely another issue with the code.
Hope that helps.
If you're using the Edit Code button here, changing anything that is not inside an editable area will break the template.
There is a warning that shows up at the top when you click into the code to edit it. A better approach here would be to go into your email template and make the change to the metatag there, then re-approve the template and the emails that use that template to push the update thru to this asset.