Media: Make the image editor help toggle icon color admin scheme-aware

[t-hamano]

The help icon color in the image editor was hardcoded with classic colors. By replacing this with a CSS variable, it will default to modern colors. Furthermore, the color will appropriately change according to the user's color scheme.

Trac ticket: https://core.trac.wordpress.org/ticket/64937

Screenshots

Note that the button is intentionally focused to visually display the focus outline.

Header	Header
Default
Fresh
Light
Blue
Coffee
Ectoplasm
Midnight
Ocean
Sunlinse

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[mukeshpanchal27]

Thanks @t-hamano for the PR!

Look good to me.

[Dervish12]

Nice fix. Replacing the hardcoded #2271b1 with var(--wp-admin-theme-color) is the right call — this makes the help toggle icon consistent with every admin color scheme instead of being stuck on the classic blue.

The focus state change is also correct — border-color and box-shadow should match the theme color, not a static hex.

One thing I noticed: the box-shadow uses 1px width while most other focus styles in the admin use var(--wp-admin-border-width-focus, 1.5px). Might be worth aligning that too, but it is a pre-existing issue and outside the scope of this ticket.

Looks good to merge.