feat: implement comprehensive activityBarTop theming and refine icon rendering

[kairosci]

Closes: #210694

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@bpasero

Matched files:

• src/vs/workbench/browser/parts/auxiliarybar/auxiliaryBarPart.ts
• src/vs/workbench/browser/parts/paneCompositePart.ts
• src/vs/workbench/browser/parts/sidebar/sidebarPart.ts
• src/vs/workbench/common/theme.ts

[Copilot]

Pull request overview

This PR adds new theme color keys to let themes customize activity bar item hover foreground/background colors, addressing readability issues when hovered items use the same foreground as the bar background.

Changes:

• Register activityBar.hoverBackground / activityBar.hoverForeground theme colors.
• Register activityBarTop.hoverBackground / activityBarTop.hoverForeground theme colors.
• Add theming-participant CSS rules intended to apply the new hover colors to the (left/right) activity bar.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/vs/workbench/common/theme.ts	Adds new registered theme colors for activity bar and activity bar top hover states.
src/vs/workbench/browser/parts/activitybar/activitybarPart.ts	Attempts to apply the new activity bar hover colors via dynamically injected CSS rules.

[kairosci]

Given:

{
    "workbench.colorCustomizations": {
    "activityBar.hoverBackground": "#ff0000",      
    "activityBar.hoverForeground": "#00ff00",      
    }
}

I see:

this applies to all icons .

[kairosci]

Given:

{
    "workbench.colorCustomizations": {
    "activityBar{Top/Bottom}.hoverBackground": "#ff0000",      
    "activityBar{Top/Bottom}.hoverForeground": "#00ff00",      
    }
}

I see: