Add DiffTextWidget colors to some palettes

[Leilei332]

Add DiffTextWidget colors to Solarized, Solar Flare, Nord, GruvboxDark, DesertSand and Twlight palette to make them display properly.

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[pmario]

If you have a look at the palette-manager edition: https://wikilabs.github.io/editions/palette-manager/
There are a lot of palettes, that have a WL at the end of the name.

All of them should have DiffText colours assigned. Some feedback would be welcome.

That's a reminder, that I should create a PR for all palettes. Since all of the core palettes have some missing elements.

[Leilei332]

There are still a lot of palettes that doesn't have diff-* indexes in the palette manager edition, like the two palettes:

• Solarized Light/Dark WL
• Solar Flare WL
• Spartan Day/Night WL

While those palettes have their own diff colors, which I think can be included in the PR:

• Nord WL
• DesertSand WL
• GruvboxDark WL
• Twlight WL

[pmario]

If they do not have the indexes in the palette, they use the default from vanilla. That is intended. But I will have a closer look. I have seen some problems with some dark palettes, where the text is invisible.

[kookma]

Add DiffTextWidget colors to Solarized, Solar Flare, Nord, GruvboxDark, DesertSand and Twlight palette to make them display properly.

Thank you for using a consistent method for adding new color values! Everywhere you have use 6 chars hex code! Keep going on!

[pmario]

I think only changing the foreground-colour is not enough. We have to take "colour blindness" into account.

Eg: this is "Nord dark WL" palette: Accessibility setting: Simulate: Protanopia (no red)

This is the same setting, that I use with my Palette-Manager edition, with the same Accessibility setting

The same screenshot for "normal vision"