Refresh board painting style
In response to #15 (comment 736882), it would be good to:
- Make the painted state of a cell more obvious. Perhaps make the background colour darker and invert the font colour, so that the un-painted cells seem a bit more ‘hemmed in’.
- Make the error state of a cell more obvious and distinct from the painted state, and stop relying on colour alone to distinguish errors. Perhaps draw a red cross across the background of the cell.
Any changes made here should ensure that:
- The board looks nice when not very many cells are painted.
- The board looks nice when a lot of cells are painted (with no errors).
- The board looks nice when there are several errors.
- All the cell states are easily distinguishable for people who find it hard to make out small details.
- All the cell states are easily distinguishable for people who are colour blind.
- It works on high-DPI and low-DPI screens.