Issue #3451667: Add default theme to AssetResolver::getCssAssets cache key-id.

Closes #3451667

added 262 commits

• e4f26ece...4d8f5eb2 - 259 commits from branch project:11.0.x
• dee1caad - issue/3451667: Added themehandler AssetResolver file.
• 3c953389 - issue/3451667: Updated core.services file.
• a06d2a36 - issue/3451667: Updated test covergrage AssetResolverTest

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