Add invert font color settings red/green

[b0bdN]

Hi,

I added the settings to change the font color for the positive and negative value since the issue #170 was mentioned. I learned that in most countries, the color are reversed, thank you @EdwinKuo for this lesson :)

I think that the code can be more efficient because I reuse the same if statement but with the color reversed.

[helmut72]

Would be also cool to have a pure system color mode (instead red and green). I've problems to read red colored text. Thank you.

[weyheng]

eXtra-Large for widget text size?