Make background color of GTK+2 progress bars consistent with GTK+3

[livanh]

I noticed a difference between GTK+2 and GTK+3 in the derivation of the background color of progress bars. This is not apparent in light themes but quite visible in dark themes. So I changed GTK+2 to use the same method as GTK+3.

[livanh]

I am still a little unsure, maybe a few screenshots can help us.

This is how GTK+3 looks on master with a dark theme:

And this is GTK+2: I remember that it used to look similar to GTK+3, and I preferred it that way (or it's just my resistance to change).

The odd thing is that if I edit GTK+3 with "background-color: mix($bg_color, $base_color, 0.08);", which I think should be the translation from GTK+2 syntax, it still looks very similar (and different from GTK+2). Instead, with "background-color: mix($base_color, $bg_color, 0.08);" it looks similar to GTK+2:

What is the intended look w.r.t. these screenshots?

[actionless]

2nd and 3rd screenshots are looking fine

so yeah, current gtk+3 need to be fixed

[actionless]

regarding your last point: i think the order of arguments is different in mix functions of gtk2 and gtk3

[actionless]

thanks!

now i see travis complains on scss codestyle: https://travis-ci.com/themix-project/oomox-gtk-theme/builds/110622532

and also i guess the screenshots would need to be updated to represent the change, https://github.com/themix-project/oomox-gtk-theme#generate-new-screenshots

# start container mounting the current dir:
./docker_scripts/dev_docker_container.sh
# Generate new screenshots:
./docker_scripts/run_docker_tests.sh -e GENERATE_ASSETS=1

(i know it sounds a bit boring -- but at the end we'll be able to see the visual diff on github, for 3 themes, light, medium and dark)

[actionless]

rich diff on github rules :3

now we can definitely see what your fix was effective :)

thanks a lot for all your effort!

[livanh]

You're welcome! I didn't know about the rich diff, it's nice indeed!