Spoiler option?

[FatFork]

If you post text with "transparent" color, anyone reading your post will not know there is text hidden unless you say so in your post.

Potential solution:

Spoiler: Spoiler text.

Code:

Spoiler: [u][color=transparent]Spoiler text.[/color][/u]

The best part is that this solution also works on Ninjastic.space (both light and dark themes).