You are indeed able to develop new themes as long as you and your team have advanced knowledge of JS/CSS. 

On the last page in your Customizations course, titled "Theme Colors", you will find a Theme CSS Editor. This page allows you to create custom CSS themes. Manipulating the code on this page will be live reflected in the background. Once you are happy with your results, you will copy your completed code, upload it to the canvas_global.css file in the Canvas theme editor, and then add a new row to the Theme Configurations table in your Customizations course to make your new customized theme available.

We do have to warn you that there are many issues you may run into with the responsive nature of themes. It is important to note that Cidi Labs does not provide training on custom CSS, nor do we provide support for any custom CSS. This means you will need an expert in-house to create and maintain your custom themes. Maintaining includes fixing any code that Canvas breaks during their regularly scheduled updates.

Another aspect to consider is that the next generation of DesignPLUS will offer simpler ways to create customized themes. The theme building interface will be more of a WYSIWYG style editor. This will make developing custom themes much easier. To stay up to date on the progress of the new generation of DesignPLUS I recommend following our monthly release notes.

If creating a custom theme is something that you and your team are still interested in pursuing, we are happy to schedule a call with our development team. They may be able to give you a few tips to look for as you are building out your custom code.