Poor diff highlight contrast in Dark Theme (Compare editor and Unified Diff) #2993

Problem

Users reported that in Eclipse Dark Theme, the changed-line highlight colors in the Compare editor have very poor contrast: the red/green diff backgrounds are barely distinguishable from the near-black editor background. Other tools (e.g. VS Code) achieve noticeably better contrast for the same type of diff.

The problem affects both:

• The 2-way Compare editor (TextMergeViewer)
• The Unified Diff view (inline code mining annotations)

Root Cause

The default base colors are defined in plugin.xml (lines 82–108):

• DELETION_COLOR = COLOR_RED (#FF0000)
• ADDITION_COLOR = COLOR_DARK_GREEN (#008000)

These are then blended 90% toward the editor background by TextMergeViewer.ColorPalette:

normal   = interpolate(selected, background, 0.6);
fill     = interpolate(selected, background, 0.9);  // used for line background
textFill = interpolate(selected, background, 0.8);

On a dark background (~#1E1E1E), a 90% blend of dark green/pure red toward black produces a color barely distinguishable from the background. The same logic applies in UnifiedDiffCodeMiningProvider (interpolation at 0.9/0.8).

Existing Partial Fix

dark.css (team/bundles/org.eclipse.compare/resources/css/dark.css) already overrides the unified diff annotation colors for the dark theme:

IEclipsePreferences#org-eclipse-ui-editors:org-eclipse-compare {
    preferences:
        'inlineDiffColor=27,40,30'
        'detailedInlineDiffColor=24,50,27'
        'deletionInlineDiffColor=52,27,30'
        'detailedDeletionInlineDiffColor=74,24,27'
}

However:

1. These values are still very dark — the colors are slightly less wrong but still low-contrast.
2. The 2-way Compare side (ADDITION_COLOR/DELETION_COLOR in the JFace color registry, consumed by TextMergeViewer) has no dark theme override at all.

Light-Theme Defaults (for reference)

Affected Files

Proposed Fix

1. dark.css: Raise the brightness of the 4 annotation color values so they are visible on dark backgrounds (e.g. 60,90,60 / 100,50,55 instead of 27,40,30 / 52,27,30).
2. plugin.xml: Add dark-theme overrides for ADDITION_COLOR and DELETION_COLOR (via themeElementOverride or by adjusting defaults) so that TextMergeViewer's interpolation produces a visible result on dark backgrounds.