Rename CSS variables and improve colorblind themes

[silverwind]

Followup #36215, rename the variables for consistency with existing vars and change green to value of --color-blue in the relevant color blind themes:

The blue coloring also matched GitHub:

[wxiaoguang]

It seems that GitHub uses orange but not red

[silverwind]

We could potentially change our red shade in colorblind to a bit more orangey and then also add a color with same hue for --color-diff-removed-fg.

[silverwind]

Some tweaks done, still wip. What I don't quite understand is the difference between the two existing colorblind themes. From master:

red-green:

blue-yellow:

The only difference is slightly darker red. Both variants can distinguish orange from blue, so I think we can make them look exactly the same.

[silverwind]

Ok should be ready now. Previously the blue-yellow theme imported red-green and then reset red to original red, now I removed this import between the colorblind themes and override only what is actually needed. Here are all 4 themes:

In the future, we could use https://caniuse.com/css-relative-colors to automate such hue rotation, but currently browser support isn't there yet.