MyBB is a very complete forum software, and has a light and complete theme system and template engine that allows you to install themes easily and without having to modify any code. Find some MyBB themes in the official themes section.
Download your theme and decompress it to a folder.
Upload all files located in the upload/ directory into the /opt/bitnami/apps/mybb/htdocs directory.
Log in to the Administration Control Panel and navigate to “Templates and Style”. Click the “Import a Theme” tab.
Click “Import From -> Local File”. Find the folder where you decompressed the theme’s folder and look for a .xm file. Select this file.
Give the theme a name - for example, the official name of the theme you downloaded.
Import all stylesheets and templates, and set the MyBB master theme as the parent theme.
Click “Import a Theme”. If you get any type of error, check “Ignore Version Compatibility” and click the “Import a Theme” button again.
Next, click the “Templates and Style” tab. You should now see your new theme listed in the “Themes” category box.
To set your theme as default, click on your theme name’s “Options” button and then click “Set as Default”.
To force all users to use that theme, click “Force on Users”.