@xinniw on DPF (the plugin framework I created and used here for the new plugin), one of the rendering modes is using opengl, so plugging imgui for that was simple enough, check https://github.com/DISTRHO/DPF-Widgets/blob/main/opengl/DearImGui.cpp for the implementation.
I only needed to do that once, now everytime I need some imgui things is just creating a ImGuiWidget based class and do custom drawing on the virtual draw function.
See https://github.com/trummerschlunk/PodcastPlugins/blob/main/plugins/ui-widgets/InspectorWindow.hpp for the theme editor, its just a few lines