-subtle color background utility classes don't support opacity

[coliff]

Prerequisites

• I have searched for duplicate or closed issues
• I have validated any HTML to avoid common problems
• I have read the contributing guidelines

Describe the issue

It's easy to change the opacity of a .bg-secondary div with the bg-opacity-50 class.. however I noticed that the bg-*-subtle classes do not support changing the opacity. It would be useful if they did. I assume the subtle colors would need RGB values added as CSS custom properties for this to be enabled

If support for this isn't planned perhaps should add a note to the docs mentioning that bg-opacity classes don't work with -subtle colors.

Reduced test cases

Stackblitz demo:
https://stackblitz.com/edit/bootstrap-subtle-opacity?file=index.html

What operating system(s) are you seeing the problem on?

Windows

What browser(s) are you seeing the problem on?

No response

What version of Bootstrap are you using?

v5.3.0

[mdo]

Right now we're not generating any -rgb variables for these colors, but we could. That feels like a slightly larger lift than a patch release, so removing the assigned project from here for now. @coliff Do you want to take a stab at something here?

[coliff]

sorry, don't think I'll be able to do this anytime soon. I'd be happy for someone else to jump it and take this on though!