How to build a user-defined theme?

I need to allow users to define certain colours that change the look of my app. to suit them. I currently use a single less file to define the variables that represent the colours that can be changed. This less file is imported into my main app. less file using @import.
I don’t have an option to try and update this variables file as this will change the appearance of the app. for everyone. I need something that will get values stored against the user’s account in Mongo and dynamically add them to the app. to change the colours of the app. - any ideas?