The dark component out of the light-dark() of
--color-message-list-border is hsl(0deg 0% 0% / 40%), which is the same
as the color we specify in dark_theme.css.
- .recipient_row already uses the above variable, so it's entry is not
needed in dark_theme.css
- .message_row uses the same variable as well.
- .overlay-message-row does not have a direct border. Although
.message_row nested inside it does. It already uses the above
variable, so we can remove this entry as well.
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