I recently created this tab / sub-tab Qlik Sense extension to improve navigation between sheets. Sub tab grouping helps organise sheets when there are a lot of them.
In the screenshot example:
Each sheet (sub tab) has a label and is assigned a grouping (top tabs). Colour schemes can be defined within the extension. I recommend setting the extension as a master item and re-using across your app.
Please feel free to contribute to the project on Qlik Branch. The code is simple to follow and could be used as a good introduction to building extensions.