Cosmetic text is a reward players get for completing the right hand side of the Raid Quests for the Raid: The Smugglers Are Back.
Once players complete the quest: Heart of Home, which is only available while this raid is running, players will unlock a new feature they can access via their settings page.
On your settings page you will see, to the right or bottom for mobile, a cosmetic text section.
The gist of this is to allow you to select the color, bold and italic to allow you to stand out a bit in chat.
Here you can see we chose a color, we set the text to bold and italic – see image to the right or bottom for mobile.
I chat, you can see that when we type anything – our text reflect our choices. See image to the right or bottom for mobile.