Dark theme. Soon to be made runtime switchable (instead of needing to find the bit in the source code, change it, and recompile).
Also dashed lines with a drop down to chose the pattern (not depicted in the attached image).
Got a button that switches the graphics display portion of my program, but I can't figure out how to set the Gtk Theme Variant at runtime within my code. My system is using a dark theme, and launching with GTK_THEME=:light seems to adjust it. Found some code from https://github.com/electron/electron/commit/058d0bbe19a61b0709ac75cb258ee65147d9ac57 but it doesn't seem to work (even after I adapted it to gdkx).