Updating gtk-2 assets and color variables

[Murat-Karakaya]

As metioned in the #511 issue, the themes of gtk-2 and gtk-3 apps are not consistent. The changes made to make them consistent only affects the hex color codes and the rest of the code remains unchanged. I tried to test them as much as I can. And even forced the gtk-2 themes to Qt apps to see how well the new theme behaves over there. So far I couldn't see any issues.

The pull request doesn't has the updated asset png's nor does it has the updated variations. I didn't staged those parts to avoid making a massive and unauditable PR.

Here are some screenshots of gtk-2 only apps with the new themes:

[Murat-Karakaya]

I want to point out that the old gtk-2 color theme used to have a color inconsistency in specific entries that are common in font selections. I thought that this wasn't made on purpose so I didn't replicate it on the new theme in this PR.

current gtk-2 theme

gtk-2 from this PR

[Murat-Karakaya]

There is one issue however, The insensitive disapled inputs have slight color inconsistency with entry itself and the padding.

I made some more changes that fixed the mentioned inconsistency

I also fixed the scrollbar and the checkbox inconsistencies.

[Murat-Karakaya]

New assets and variations are updated in master_build branch for those who want a quick test

[clefebvre]

Thanks for doing this.

I'm leaving it open for now because there's an open issue on #510 which might affect some of the GTK3 colors.