Allow high contrast themes for WEBTHEME

[rdwebdesign]

Description

Fix #1337

Motivation and Context

The validation function wasn't allowing the recently introduced themes.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[sonarcloud]

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0 Vulnerabilities
0 Security Hotspots
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