style(generatescssfromtoken): add common option to md2 token

[Jason20020]

Description

Add a third option "common" to replace the same value of "light" and "dark" token. Reduce the var of scss.

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots:

Before (If applicable):

After:

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 change)
• [ ] Documentation update
• [ ] Code style update (formatting, local variables)
• [x] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Other... Please describe:

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.