elementary high contrast (light) stylescheme

[jeremypw]

Fixes #548

Based on the elementary-light theme but tweaked to produce better contrast, using the elementary palette mainly.

• Make most colors darker/more saturated
• Make most backgrounds lighter
• In some cases make style bold
• Improve contrast between line number panel and text panel background (which is what the issue was about)
• Make the current line background different from the text background

Preferably needs testing by a user who prefers or requires a high-contrast theme, and on as many different languages as possible. I checked on Vala, C, XML and Meson mainly.

The theme probably works better with a bold font but for comparison with "classic" below, the same regular font is used.

Comparison between "elementary-high-contrast" (on the left) and "classic" scheme (on the right)

[jeremypw]

Putting up for review to see whether this has any legs. I have used it for a while without problem.

Do we need to give option for the "classic" high contrast scheme as well?

[zeebok]

I like how it looks personally but I don't know if there are any UX concerns with it besides the contrast meeting standards

[jeremypw]

Requested UX input from @danirabbit

[danirabbit]

I thought it was maybe @micahilbery who did these for Terminal? Probably would have good insight

[jeremypw]

@micahilbery Would you like to apply your expertise when you have time? Thanks.

[micahilbery]

Overall I like the direction, just a couple comments.

[jeremypw]

@micahilbery I've incorporated your helpful suggestion, thanks. Because the elementary palette appears less saturated than the classic one I think the theme works better with a bold font but I am not sure whether we should impose that.