Mercury | August 2017
Mercury is a Cydia tweak that adds a customizable indicator to messages that you haven't responded to. I'd been using it personally for a bit over a year now, so figured it was time to polish it up a little and release it. I have developed a less fully-featured Mac version as well, which you can download/install using SIMBL (found in this repo).
The tweak is written in Objective-C using the Theos development suite, specifically Logos that allows you to hook into iOS functions and modify how they work. It uses a repo called libcolorpicker to be allow high customization of the indicator.