The differences between squares that have not been touched yet and squares
that have no mines around them are subtle.  I personally, at least, have a
hard time telling at a glance where the border of the unexamined squares is.

Under examination, of course, it is possible to tell, but I am often
confused for a second after moving my eyes quickly on the board. 
Especially when one is trying to play fast for a high score, the more
obvious what each square is, the less likely one is to make a mistake.  I
feel that the color or shading could be made more pronounced.

Some desktop themes make this more or less of a problem.  As an example of
a large difference, use the High Contrast theme (as expected).  As an
example of a small difference, use the Simple theme.

As a suggestion, could we maybe leave out the hyphenated grid lines?  That
would make the cleared sections seem more of a 'wasteland' where clicks
don't do anything and which the player needn't worry about.