Battery widget: text is hard to read in colored (green) mode

[fengjiawang]

Hi maintainers,

Thanks for the great project!

When the battery widget is set to the colored style, the text/percentage font color stays black, which makes it a bit hard to read due to low contrast.

Would it be possible to:

1. support customizing the font/text color for the battery widget, or
2. change the default text color in the green/colored background to a more readable color (for example white)?

Thanks!