Tweak checkerboard colors to be easier on the eyes

[Calinou]

The colors match GIMP's #666666 and #999999 and are easier on the eyes (thanks to reduced contrast), especially on a dark theme.

This also inverts the pattern in Checkerboard.svg to match the other checkerboard icons (top-left square is dark).

Preview

Dark theme

Before	After

Light theme

Before	After

[KoBeWi]

I wonder if we need 3 checkerboard images 🤔

[Geometror]

While the reduced contrast may be easier on the eyes, I’m concerned it could impact functionality. The lower contrast makes it harder to accurately assess the actual color, which is crucial in this context. I think functionality should take priority over aesthetics in this case.

[SpockBauru]

Not sure if its my monitor or my old eyes, but I find the "before" checkerboard easier to see...

[Calinou]

The lower contrast makes it harder to accurately assess the actual color, which is crucial in this context. I think functionality should take priority over aesthetics in this case.

I tend to assess the color relative to how most other software presents it, and Godot regularly throws me off here. It makes semi-transparent colors seem different to other software, usually making me think the color is less opaque than it actually is.

Not sure if its my monitor or my old eyes, but I find the "before" checkerboard easier to see...

The old checkerboard is actually easier to see (since it has higher contrast), but that can be a problem at times since it can be distracting.