use automatic colors that contrast with WindowBg

[pthom]

Based on modifications first added into HelloImGui: https://github.com/pthom/hello_imgui/commit/425490c622fa7527bd069a2c68ea641154369f8d

See discussion at https://github.com/pthom/imgui_bundle/discussions/72

Beware, this PR was not tested in the ImGuiAl repository; this is a direct port of the modifications.