[web-php] PR #689: Use CSS variables for some common colors

Pull Request: https://github.com/php/web-php/pull/689 Author: morrisonlevi Aside from being able to tell what color it is or what purpose it solves just by looking at the variable name, it helps provide a foundation for more easily changing the theme in the future. Hopefully, we'll have a light and dark theme one day :fingerscrossed: Mostly, colors are unchanged. However, I did change some colors which were close to the theme colors but not quite.