Improve range-diff color contrast in dark mode

The current colors are very low contrast, and therefore very hard to
read. This replaces the colors with significantly brighter ones to
improve contrast a lot.

The red drifted a lot into the orange, because when making the red
brighter, it just got *harder* to read for me because red on black is
bad combination. So i just made it slightly orange, which looks a lot
better. I did not pick the colors from `git range-diff` itself because
the red there is still pretty low contrast.

The colors now have a comfortable ~4.55 (AA accessibility) contrast
according to developer tools.

Author: Noratrieb

src/gh_range_diff.rs

@@ -303,10 +303,10 @@ fn process_old_new(
         color: rgba(0, 255, 0, 1);
       }}
       .line-removed-before {{
-        color: rgba(100, 0, 0, 1);
+        color: rgb(206, 48, 0);
       }}
       .line-added-before {{
-        color: rgba(0, 100, 0, 1);
+        color: rgba(11, 142, 0, 1);
       }}
       .word-removed-after {{
         color: black;