Getting `load-theme` to work in `-batch` mode?

[Konstantin Kharlamov]

I'm writing tests for color-identifiers-mode, and as part of the
testing I need to check it reacts correctly to theme being changed.

There are already tests that work fine so far, but it turns out `load-
theme` doesn't work as is. More specifically, even though `(load-theme
'adwait)` works fine, but background/foreground have no color:

   emacs -batch --eval "(progn (load-theme 'adwaita) (print (face-
   attribute 'default :foreground)) (print (face-attribute 'default
   :background)))"
   
   "unspecified-fg"
   
   "unspecified-bg"

I asked on Emacs stackexchange¹ and I was hinted that it's because
batch-mode doesn't initialize a windowing system.

But is there no function I can call to make `load-theme` assign colors
as in non-batch mode?

P.S.: pls keep in in CC

1:
https://emacs.stackexchange.com/questions/80358/unspecified-fg-bg-in-batch-mode-when-querying-foreground-background-colors/80370#80370