Set the correct color for popover header bottom border

[brneor]

Fixes #34005

[ffoodd]

Not quite sure about this: the existing border color is meant to be a shade of the background color. That's questionable and I tend to agree with your suggestion, but it's more a design decision than a CSS one. @mdo, your call :)

[brneor]

Not quite sure about this: the existing border color is meant to be a shade of the background color. That's questionable and I tend to agree with your suggestion, but it's more a design decision than a CSS one. @mdo, your call :)

I was in doubt if this was a flaw or a design choice, but as I stated on #34005, it differs from the card-header behavior.

[ffoodd]

Well, they aren't the same component, so they may differ :D In this case, popover lies above another content, so its surrounding borders are more important than card's ones IMHO. But I agree it'd be more consistent, even in the component scope.

[brneor]

Looks like pull bot messed up this PR...

[XhmikosR]

@brneor please make a new PR and ideally don't use that bot please.

[brneor]

@XhmikosR done in #34026. Sorry for the inconvenience.