cycle-theme doesn't work on newer versions of Doom Emacs

[janusvm]

Running jsvm/cycle-theme (SPC t t) does not work consistently across versions;

• Doom v3.0.0-alpha on Emacs 27.1 (WSL): works :heavy_check_mark:
• Doom v21.12.0-alpha on Emacs 28.0.50 (WSL): doesn't work :x:
• Doom v21.12.0-alpha on Emacs 29.0.50 (native Linux): doesn't work :x:

On the latter version, I also get an error when trying to run describe-variable (SPC h v) on doom-theme after running jsvm/cycle-theme:

elisp-refs--read-all-buffer-forms: Unexpected error whilst reading ~/.emacs.d/core/core-ui.el position 229: (void-variable read-symbol-positions-list)

It's likely that the implementation of jsvm/cycle-theme messes up the doom-theme variable, as right now it works by hard setting the variable and calling doom/reload-theme. A better implementation would probably be to simply use load-theme directly.

[janusvm]

The void-variable error turned out to be due to read-symbol-positions-list being removed in Emacs. It can be fixed by defining the variable myself somewhere.