I suggest that you always install the v2.9.8 Release via IDE Library Manager.
"FreeSevenSegNumFontPlusPlus.h" is in the current Beta on GitHub.
Your project should not care whether it is present or not.
I will try the GitHub - Ed-EE-Eng/Graphics_Thermometer: Arduino MEGA Graphing Thermomoter Project Using 480x320 LCD project later.
David.