zPages: Improve the color and style to be consistent with Java.

[songy23]

@adamgarza has done a bunch of improvements on the Java zPages, and now the zPages look like this: https://github.com/census-instrumentation/opencensus-java/tree/master/contrib/zpages#view-stats-and-spans-on-z-pages.

We should apply the same color and style to zPages in Go.

[semistrict]

Can we extract the CSS+HTML and reuse it rather than hard coding it again?

[songy23]

In Java we found it's difficult to use a css file so we stored the styling in a Java string:
https://github.com/census-instrumentation/opencensus-java/blob/master/contrib/zpages/src/main/java/io/opencensus/contrib/zpages/Style.java#L22

It should be trivial to just extract the styles and reuse them in a css file.