Swapping themes?

[br8kwall]

Looks like a lot of work went into developing the Modern UI. Very modern. Congratulations!

What are the plans for supporting alternative themes? Is it just to change out the contents of the CSS directory? For example, if I have some CSS that gets the 1.3.0 version where I like it, can I just replace the contents of CSS/ with my 1.3.0 one?

Is there any general guidance on do's/don't/approaches for developing new themes?

[atrol]

What are the plans for supporting alternative themes?

No plans at the moment

LIs it just to change out the contents of the CSS directory?

Yes in principle

can I just replace the contents of CSS/ with my 1.3.0 one?

This will not work as the new design is a complete different thing

Is there any general guidance on do's/don't/approaches for developing new themes?

No
You should be aware that the new CSS is much more than just some new colors, backgrounds, positions, ...

The new UI is a so called "Responsive Design" so Mantis can be used on phones, tablets, notebooks and desktops.
The UI automatically changes depending on the device you use.

If you want to develop another theme, you need knowledge of HTML5, CSS3, SASS, JavaScript, bootstrap framework and working with templates like this one
https://github.com/bopoda/ace

So the good news is, that Mantis will come with a quite state of the art modern UI.
The bad news is, that it will be harder for hobby developers to change the UI.

[br8kwall]

I was hoping to make a new lighter color version of the CSS of the modern theme from my Github fork so that I push stuff back up. But I'm getting bitten by the phpMailer autoloader shown in the below issue if I deploy directly from git.

https://www.mantisbt.org/bugs/view.php?id=21413

Is there a work around for this?

[atrol]

GitHub delivers just the source of your repository when downloading a ZIP-file.
You have to implement your own packaging mechanism if you need more than this (the submodules in your special case).
This is what we do with this tools https://github.com/mantisbt/mantisbt-tools

You might have noticed that downloading Mantis from GitHub as a ZIP file does also not include the Mantis documentation.