feat: Added "halloween" streak stats theme

[DenverCoder1]

Description

Added "halloween" theme.

The inspiration comes from the GitHub Halloween contribution graphs that have appeared on October 31 each year.

This is an opt-in theme and can be used by setting the theme parameter to halloween.

Type of change

• New feature (added a non-breaking change which adds functionality)

Checklist:

• I have checked to make sure no other pull requests are open for this issue
• The code is properly formatted and is consistent with the existing code style
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Screenshots