CSS: fix .unclicked :hover behavior

Use the unclicked class for the polygon group, and add selectors to
excluded flagged and bomb classes.
CSS: add gradient for .unclicked :hover

Use a radial gradient instead of flat white for hovered polys.
Hex,Pen: decimal consts to algebraic expressions

Instead of using opaque strings of digits, express constants as
algebraic expressions to ease future manipulations.