Have you gone over the code for your game? I really love how the gui was built but could only guesstimate what code was used.
Viewing post in Devlog#4: GUI and music for A cup of eternity comments
Hi momo, depends on what aspect you're interested in specifically?
For the lantern sway on main menu, it was a custom shader by yours truly. Actually, for a previous project (The Man of my Dreams), I had already coded a custom grass sway shader. So the lanterns swaying was basically taking that grass shader (top moves, bottom of the image stays fixed) and reversing everything. Formula consists of quite a few cos and sin strewn around and playing with them until it looks right.
For custom menu buttons, it's about replacing textbuttons from default menu code by imagebuttons and adding actions to them depending on how you want them to react (cf official Ren'Py documentation for the syntax).