How to add this (screenshot attached) beside thread's Title?

When editing category in Misago admin panel, you can enter css class name. Try one of those:

• red
• light-red
• pink
• light-pink
• purple
• light-purple
• deep-purple
• indigo
• light-indigo
• blue
• light-blue
• cyan
• light-cyan
• teal
• light-teal
• green
• light-green
• lime
• yellow
• amber
• orange
• deep-orange
• brown
• light-brown
• blue-grey
• light-blue-grey
• grey
• black