But what does it mean?
Click and drag to move pieces
A red outline means the active piece is in an incorrect place
Dark pieces need to be flipped (or rotated, if you're using a touch screen) until they turn bright
A green outline means the active piece is in the correct place
Double-click a piece to rotate it by 90 degrees
Click the middle mouse button or mouse wheel to flip a piece along the Y-axis (does not apply to touch screen users)
Right-click to flip a piece along the X-axis (does not apply to touch screen users)