Make Slack selector opacity 1.0 by default, or allow custom styling

[michaelshum]

Not sure if this is a bug or a feature, but the slack selector is default transparent until hovered over. The Slack default is that "Add reaction" is visible now, it'd be great to be able to follow suit OR allow custom styling to override

[charkour]

Hey @michaelshum, I think allowing custom styles to override would be a great idea.

Would you be willing to make a PR? Thank you!