The FluidTopicsThemeService.getActiveTheme() JavaScript method provides information about the current active theme for the user.
Example
The following example prints the current active theme for the user in the user's browser console.
console.log(FluidTopicsThemeService.getActiveTheme());
{
"id": "8a317675-3109-4957-bfdc-bdc8d67eb5f3",
"name": "Dark theme"
}