[Popover] Tip triangle should have `--highcontrast-` values in color applications.

[Westbrook]

spectrum-css/components/popover/index.css

Lines 84 to 85 in 8214cdf

	fill: var(--mod-popover-background-color, var(--spectrum-popover-background-color));
	stroke: var(--mod-popover-border-color, var(--spectrum-popover-border-color));

The above can lead to

Seems like there are system values that should be leveraged ahead of the --mod- values to make sure the force colors are delivered appropriately here.

[pfulton]

Added to our Jira backlog. Thanks for reporting this.