The Potlatch school colors are Forest Green and white.
The programmer didn’t choose that theme. It was made by assignment and historical. White (or off-white to creme) is a given for almost all themes. Does the school print green text on white background? Or white-on-green? I bet they use both depending upon the application.
Potlatch uses what is called a “hanging curtain” style page layout. This is conservative. It is a fixed-width technique and universally adopted.
Any reason for not choosing more fluid elements with full-width percentage?
Fluid is a way to distribute color more evenly in cross-device modes when screen sizes vary. Liquid or fluid design maintains the “theme” color ratio / hierarchy. Most people aren’t aware of this subtle benefit.