Is your feature request related to a problem? Please describe.
I often make a spell or an item glow on the action bar in my WAs. For that, I use the Glow External Element → Frame Selector option and choose the corresponding action bar button where the spell is located.
Problem is, depending on my current talent loadout, the spell can be assigned to a different action button, resulting in the glow appearing on the wrong button.

Describe the solution you'd like
Add a new option Spell or Item for the Glow External Element feature in addition to the existing ones Frame Selector, Nameplate, Unit Frame. With this new option, users can input the spell ID or item ID, enabling automatic glow for the action bar button(s) that contain the specified spell/item.

Additional context
Currently, glow is limited to a single external frame. Could you consider adding the ability to apply a glow to multiple frames, including multiple spells or items with the proposed feature?

We have investigated this in the past and found that it's pratically impossible reliably map spells to action bar buttons, due to how action bars work.