Slush: Add text and link color guidelines to stylesheet

[machikoyasuda]

• The color variables and names are starting to get confusing.
• Add stylesheet examples for every text color/size and link color/size combo that is currently being used in the app.

[machikoyasuda]

Closing this as not a priority until any styles change. When/if any new styles are decided upon, we can revisit this to docment the new changes.