Add Arc-Black color variant as an option

[savorocks]

Hi,

I wanted to suggest adding Arc Black color scheme in this package as I found that it makes a pleasant difference in contrast to arc dark variation. Arc Black has been my default style for GTK, but unfortunately it is still not available through Kvantum.

According to author: "The black Arc desktop theme darkens only six elements, on the whole, but where it happens makes all the difference. Significantly, keeping the official Arc hues, contrasts, and structure wholly intact was foremost in this project's objective. Thus, nothing is altered one tiny bit in those crucial areas - as the existent, intact, Arc hues are pragmatically darkened."

This means that minimal changes were made, but the difference is not small, as can be seen in the screenshots below.

[savorocks]

I found Arc Black GitHub Page and in there a file with values that are changed. I've tried to edit the kvantum theme but with limited success, probably because values are named somewhat differently in Qt and GTK. Can somebody with more experience assist me in how to change the values so I get Arc Black theme for Qt apps? Below is the list of values changed:

gtk-color-scheme = "base_color: #21242b"
gtk-color-scheme = "text_color: #d3dae3"
gtk-color-scheme = "bg_color: #1a1d23"
gtk-color-scheme = "fg_color: #d3dae3"
gtk-color-scheme = "tooltip_bg_color: #d3dae3"
gtk-color-scheme = "tooltip_fg_color: #21242b"
gtk-color-scheme = "selected_bg_color: #4f94e5"
gtk-color-scheme = "selected_fg_color: #ffffff"
gtk-color-scheme = "insensitive_bg_color: #484c53"
gtk-color-scheme = "insensitive_fg_color: #a4aab3"
gtk-color-scheme = "notebook_bg: #21242b"
gtk-color-scheme = "dark_sidebar_bg: #13151a"
gtk-color-scheme = "link_color: #4f94e5"
gtk-color-scheme = "menu_bg: #21242b"