I installed a new theme and I'm changing the colors, what is this little blue part(where the arrows are) and what is it's class name so I can change the color?
Please give me your forum URL.
looks like you have already changed the required color.
for others who may be interested:
MyBB 1.8.x themes might consist of different color schemes.
if they are available then required color set can be selected at theme properties (in admin panel)
This looks like a border-bottom in class thead but without a forum link, we can only guess.