Go to Admin CP > Templates & Style > You will see MyBB Master Style and Default.
*Do not edit or delete the Default theme, you will regret it.
Click the "Options" button right hand side of Default > Click Duplicate Theme
Do not delete> Give your theme a name and leave the box ticked "Duplicate Templates" > Click the Duplicate Theme button.
Now you will see "Stylesheets" forget about them for the moment and scroll down to "Edit Theme Properties" You will see various options there but we will leave the setting as they are for now.
Scroll down to "Manage Colors" You will see under Base Color it says "None" that is the default as you are seeing the forum now, click it and choose a base colour. *Don't touch manage colors Click "Save Theme Properties"
Go into the forum and your UserCP > Bottom right under "Other Options" you will see "Board Style" open that and click the theme you just made by finding it's name. > Cluck "Update Options.
Now look at the forum and see what you think. If you don't like the base color you can change it by going back to your theme in AdminCP and change the base color.
If you look at the default theme you will find "Manage Colors" and you will see these:
black=Black
calm=Calm
dawn=Dawn
earth=Earth
flame=Flame
leaf=Leaf
night=Night
sun=Sun
twilight=Twilight
water=Water
This thread also helps:
https://community.mybb.com/thread-72931.html
As does this one:
https://community.mybb.com/thread-33809.html
From there it's all about playing with your theme but the basics are there for you to have fun with.
*Just remember not to mess with the default theme and keep a note of changes you make to your theme in case an update breaks it
Have fun.