Add High Contrast theme

[tfuxu]

New Preset: High Contrast

This isn't my work, files in this PR originate from issue #65 (more precisely from https://github.com/GradienceTeam/Community/issues/65#issuecomment-1326843903). I'm just putting them here, so that they can be further improved.

Description

(Preset description should be in the preset file itself BTW) A simple high-contrast theme variation of GNOME high-contrast theme. It's more contrasted from original, and utilize more borders for easier visibility.

Palette

OC, made by @daudix-UFO, from an idea by HubKing

Screenshots (optional)

High Contrast:

High Contrast Dark:

Wallpaper

There's none

Checklist

Before submitting the PR, I checked:

• [ ] ~I've only modified one preset~ (why can't I add more in one PR?)
• [x] I've added the URL to my raw preset in presets.json
• [x] I've checked the format of each json files I modified
• [x] (optional) I've added screenshots
• [x] The preset name is following preset naming guidelines.

Publication

• [x] I want to add my preset in the preset list. (obviously)

Thanks for your submission we will review and merge it as quick as possible.

[daudix]

It needs window outlines BTW, but this needs some CSS

[asutoshvariar]

Is there any way to have a high-contrast scroll bar or would that be too much for this?

[daudix]

@asutoshvariar currently it have white outline, which is more visible than the defaults, but I can make it gray in addition

[daudix]

Can we merge this @tfuxu ?

[tfuxu]

I would like to add borders to more widgets as now, some of them, because of sharing the same color with a window background, don't have any distinctive indicators that a specific area is a background and the other is a widget.

Example with TextView:

[halfmexican]

I can help with the CSS. I recently added more borders to the pop theme to increase contrast

[daudix]

@halfmexican it would be very nice!