Documentation theme for the packages of JuliaClimate

Hi people,

in JuliaDynamics where I am the lead dev, we follow the convention that all packages of the organization have the same theme in their documentation: color scheme and font type. For example compare https://juliadynamics.github.io/Agents.jl/stable/ and https://juliadynamics.github.io/DynamicalSystems.jl/dev/ .

This is done in a centralized manner by downloading css files from a repo of the organization. I think this is nice and makes the docs look better than the default theme of Documenter.jl, as well as giving a unifying theme to the whole organization and making it look more coherent.

At the moment our theme files are here: https://github.com/JuliaClimate/doctheme and you can see them in action in the documentation of the new ClimateBase.jl: https://juliaclimate.github.io/ClimateBase.jl/dev/

I've chosen the main color to be pale blue ("pale blue dot") and the accent color to be brown. Brown color is similar to the color of numbers in code blocks, unfortunately, but I still went with that instead of green, the second alternative I've been considering. The font of the docs is Lato.

Let me know what you think!

Review the existing theme files in JuliaClimate/doctheme and compare the ClimateBase.jl documentation with the JuliaDynamics examples linked in the issue. The issue is framed as a design discussion and does not identify a target package, files to change, or acceptance criteria, so clarify the desired scope before starting.