Make tutoring page color-blind friendly

[jameslzhu]

Current issue is that colors used for marking tutor proficiency with class (yellow, green, blue) are exceptionally similar in hue and brightness. For individuals with red-green colorblindness or other visual deficiencies, these colors are difficult to distinguish.

A better alternative is to additionally use border thickness / pattern (solid / dotted) to represent degree of familiarity with class.

Using bold / italics / underline does not quite establish the same hierarchy so is probably not the best option.