energy: Use text colour for the grid lines

Currently the grid lines always uses black as the colour, making them hard to see with dark themes, using the text colour instead should ensure enough contrast for them to be visible.

Bug 447970 might be relevant, although the original reporter was talking about the red line graph being hard to see rather than the grid lines.

I tested this patch on top of KInfoCenter 5.27.3 on Arch Linux, and here are the results:

Before (Breeze Dark):

After (Breeze Dark):

Before (Breeze Light):

After (Breeze Light):