Are you using Mendix version 2.4 or 2.5?
In Mendix 2.5 you can add a custom theme to the theme folder, in your project directory. When the Modeler re-deploys it removes the entire web folder. Everytime you re-deploy the modeler will use the files from the theme folder in order to re-create the files in the web folder.
In the final release the option to select a theme-package will be available again.
The tutorials on how to create your own custom theme can be found here. (MX-ID login needed)
Mendix 2.4 won't remove anything with re-deploy.
Unfortunately it's not possible yet to have a custom theme automatically being placed in the theme selector. In the forum post here, I explained how you can add your theme in the theme selector by changing the HTML page (index.html).
You are correct that we need to describe how you can add your theme to the theme selector. We are working on a How-to for this, it should be online tomorrow or Friday.
Besides the theme selector, you did get your custom theme to work correctly? I've you have some feedback about the tutorials feel free to post this. The tutorials are brand new so any feedback would be really helpful!
I made a How-to for the theme selector. Click here to view the How-to. When we release the next Mendix 2.5 version we'll have a better solution!
In the meantime I have downloaded several of the themes and created my own custom theme.
However, I cannot seem to get the theme selector to work correctly and I never see my custom theme as a selection choice.
I have reviewed the on-line tutorial without success. It is unclear in the instructions as to what goes where once the theme is completed. Certainly a SIMPLE TREE DIAGRAM of the proper folders and files expected to be in those folders would go a long way for the novice.
One of the issues I experience in developing specific CSS themes for Mendix is the lack of a visual display of the created CSS Theme. I mean the ONLY way to see the developed theme is to run it against a Mendix server. My "wish-list" for an enhancement would be a simple tool that I can use to display the CSS as I am developing and changing the look/feel. Is this possible, food for thought.