Manual:$wgShowUpdatedMarker

Details
If watchlisted page was changed after it was visited:
 * in history new edits are marked with "updated (since my last visit)" (Updatedmarker)
 * in watchlist the page title enclosed in

Also the watchlist has a message Wlheader-showupdated and a button Enotif reset that marks all entries as "visited".

Dependencies
If $wgEmailAuthentication is false, this setting is ignored.

Styling
For different styling of watchlist entries see w:WP:Customizing watchlists.

Note that  is also used to bold all watched pages in Recent and Related Changes. If you only want to change the styling for watchlist use rules like