fix issue#61 by removing font-size from .highlight
fix issue#61 by removing font-size from .highlight

issue#61 happened because the `.highlight` class was enforcing
`font-size` as 12px. after removing `font-size` from `.highlight`, the
`font-size` should inherit the value of 90%.
moreover, it looks like the entire section of `.highlight` where
`color:` has been mentioned is no longer necessary because Hugo now has
Chroma syntax highlighting so we don't need to rely on custom syntax
highlighting.

ribice/kiss,ribice/kiss