Themes: Fix disabled button semantics and focus visibility in theme installer

[SainathPoojary]

This PR fixes two accessibility issues in the Theme Installer related to disabled buttons and focus visibility:

1. Disabled semantics: Replaces the visual-only .disabled CSS class with proper semantic attributes (disabled for <button> elements, and aria-disabled="true" tabindex="-1" for <a> elements). This removes them from the tab order.
2. Invisible focus ring: Updates the focus ring color to #fff for secondary buttons on the active theme card. Previously, the default focus ring color blended exactly with the theme name background, making it invisible when reached via keyboard.

Trac ticket: https://core.trac.wordpress.org/ticket/66038

Use of AI Tools

AI assistance: Yes
Tool(s): Github Copilot
Model(s): Claude Sonnet 4.6
Used for: Edge-case checks, and assistance with the PR description; implementation and testing were reviewed and verified by me.

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