Enhance contrast on Modern theme

[YeldhamDev]

As per @passivestar's recommendation (#112258 (comment)), this changes the default base color from #252525 to #272727, and the contrast from 0.3 to 0.35.

[DeeJayLSP]

Unpopular opinion but, like in the comment I wrote for the main PR in which I compared main colors of other engines + popular what if neutral popular color themes, I would suggest bumping it up a bit more to #292929:

PR	#292929	#292929 with contrast 0.37

PR feels good regardless.

[passivestar]

@DeeJayLSP it's hard to say without using it for some time. Staring at this theme for 2 years gave me a lot of time to do a bunch of micro-adjustments to different controls, and stretching all values further into grays without looking at it for more than a couple of times is a bit risky because if some elements aren't getting multiplied by contrast properly it'll take time to notice.

It's also been tested and approved by people as a mostly dark theme, given that godot has both flexible customization controls for base color and contrast, as well as "light" and "gray" color presets (defaults for which could also be tweaked), I think it makes more sense to keep this theme more on the "dark" side.

So I'm not saying that your opinion is wrong, but rather that I just have more confidence in these values.

[Repiteo]

Thanks!