refactor: rename private property _theme to __theme

[vursen]

Description

The private property _theme that Flow uses internally for storing the menu bar item theme must be renamed to __theme in order to prevent it from possible interference with the web component's property _theme that has recently appeared in vaadin/web-components#3529.

Type of change

• Internal

Checklist

• I have read the contribution guide: https://vaadin.com/docs-beta/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

[sonarqubecloud]

SonarCloud Quality Gate failed.   

4 Bugs
0 Vulnerabilities
12 Security Hotspots
840 Code Smells

No Coverage information
5.0% Duplication