You may not be able to change the horizontal and vertical grid lines to a different shade of grey or a different color, but you can experiment with the background colors used in each type of account register. This might make for a better contrast to the light grey.
Go into Edit / Preferences / Register and click the Colors button. Select the type of account (Spending, etc.) you want to change and select a color other than the Default color. See if that works better for you. Sometimes it also works if you change your monitor settings for color, contrast and brightness.
You may not be able to change the horizontal and vertical grid lines to a different shade of grey or a different color, but you can experiment with the background colors used in each type of account register. This might make for a better contrast to the light grey.
Go into Edit / Preferences / Register and click the Colors button. Select the type of account (Spending, etc.) you want to change and select a color other than the Default color. See if that works better for you. Sometimes it also works if you change your monitor settings for color, contrast and brightness.