UIO docs should include a section on styling against UIO's injected classes

[jobara]

In the UIO docs, there is no explicit mention on how to do styling with UIO in general.
There should be a section in the "Working with UIO" doc that explains how styling customization can be done by styling against the injected UIO classes such as .fl-prefsEditor-separatedPanel.

Additional context or notes

Originally filed as FLUID-6351 by @jhung on November 9, 2018.