Update Layout Explorer Flex Axis Buttons with hover state color

[raison00]

On the DevTools Layout Explorer, to improve clarity and usability for both Main and Cross Axis, colorize the start / buttons on hover to indicate clickable state.

• colorize the Axis start & buttons #009aca on Layout Explorer Main and Cross Axis on hover

June 25, 2020 Video Review: Feedback provided by thehappyharis identifies this friction point

currently, no hover state:

with hover state applied: #009aca on hover

[kenzieschmoll]

The dropdowns have standard hover behavior from material. Closing as not planned.