Programmers usually think of theming as “skins” or “templates”. Theming in the graphic design world is NOT the same meaning.
Theming is a combinations of elements like symbols, colors, type, etc that generate user emotion. Some things aren’t appropriate for the subject matter and will confuse or frustrate the user.
These “manipulated” emotions are based on subconscious memories. We are reminded of something pleasant or bad from our past experience –within the 50 milliseconds. Our brains are hardwired to bypass reason when judging site “body language”. Our new experience is biased by old experience.
Emotional design cues (theming) in marketing is called “branding”. Same thing. Just different labels.
Color is the most fundamental and powerful component of theming and branding. From a bandwidth standpoint, color is also the web “visual asset” most easily leveraged for cross-device optimization. Color has free speed. We can specify hex code to colorize HTML and CSS. Weightlessness –unlike GIF and JPEG color images.
But color strategy and selection for web programmers is usually mysterious. Coders frequently just copy ideas from a favorite site (reverse engineering). That’s okay but hardly original. Good branding requires originality.
One sole color is usually not considered a theme. But a monochromatic color scheme is okay. Shades of a color can work. For example, emphasizing “planning” with a “blueprint” metaphor. This would use different shades of blue. But the best themes are built using color combinations.
There are mathematical methods of deriving pleasant combination of color with color wheels. These don’t take into account human psychology. An example would be selecting “red” as a dominant color. The mathematical complement for best high contrast would be “green”. But this math derived color theme with a balance of green and red combination just won’t work. Memories will instantly be triggered saying, “Christmas!” Unless of course, you are building a Santa site. :)
I’ve created some rudimentary reverse-color-lookup tools to make color selection easier based on the “emotional / feeling goals. This is UX.