Carbon model HTML report could use a visual refresh

[emilyanndavis]

The HTML report generated by the carbon model is perfectly functional, but it looks a little dated. Specifically—from a purely visual standpoint—it doesn't appear to be related to the current Workbench.

Updating the text styles and text/background/border colors to align with current branding/Workbench styling wouldn't take much effort, but it could help create a more seamless user experience.

Ideally, we'd make use of CSS variables that are shared by the Workbench so that the carbon model HTML report—along with any other HTML documents generated by InVEST models in the future—can keep up with future styling/branding updates with minimal effort.

[emilyanndavis]

I spent some time thinking through how we might export styles from the Workbench in a form that can be imported by the carbon module, but after chatting with @davemfish, I decided implementing such an approach would be over-engineering at this point. The carbon model is the only model that currently generates an HTML report, and in the future, there's a non-zero chance we will render such HTML reports directly in the Workbench rather than generating standalone files. We'll definitely keep modularity in mind as we plan for future updates, but for now, I'm just going to update the styles that are hard-coded in the carbon module.