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]

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.

[Murat-Karakaya]

I have changed this PR to a draft because the recent changes revealed some issues such as the fact that toolbar entry backgrounds were vastly different than their usual counterparts.

Other than that, border colors of most entries were changed to match the gtk3 border styles. This helped the gtk2 component to create a better contrast to the background just like in the case of gtk3 apps. However, this also came at a cost of 53 lines of change with minimal visual significance.

Background of the assets-dark.svg was changed to be the same as the gtk-2 app background to better showcase the assets look in the gtk2 apps at a glance.

[Murat-Karakaya]

With the new rework, I have finalized my changes I would make to this PR. I have purposefully left out some parts unchanged tough. Again, for a quick look, you can look into the master_build branch where I have mostly updated variations and assets. I don't add them here to not create an unauditable PR.

At lines 4730, 4662 the border hex color codes #252525, #232323 respectively remain unchanged because of minim visual significance

Assets for arrows and handle bars remain unchanged. However due to hue changes in the other assets, there is possibly now less color contrast between the arrows and handlebars with the rest of the assets. I think handle bar and arrow assets don't require changes. But I'm open to suggestions.

Currently they look like this