There are a number of Email client apps on the play store that have used the open source K9 app as the core code (for example LightMail), and then just made some aesthetic changes like colours, field views and layout, to end up with a new app.
This is what I am wanting achieve also. A new app, using K9, with some layout alterations and visual alterations.
I will provide a list of items, not too big, so we can get to a Stage 1 version of the app.
Once it is converted, then I will open a new job for Stage 2. This will involve adding in some other code I have here already to do with security, we can discuss that later.
So, this job is just for Stage 1 - changing the look and feel of the open source K9 app available on Github, and adding a few simple visual features.
K9 App - https://play.google.com/store/apps/details?id=com.fsck.k9&hl=en
LighMail App - https://play.google.com/store/apps/details?id=com.qiduo.mail