Picking an emacs colour theme

TL;DR: Here're a couple of handy functions to quickly try out all the dark-mode / light-mode themes you've already got installed, and pick the one that feels right for now. There are too many choices I know that emacs can be as pretty as I want it to be, but when I run M-x load-theme TAB I just see a big list of more-or-less meaningless names: Type M-RET on a completion to select it. Type M-<down>…