Fix: resolve absolute path for theme.custom_dir

[KyleKing]

fix: #65

Resolve the full path to a custom theme directory. Fixes the issue that specifying a relative path would cause the configuration to fail

The code changes are covered with hypothesis testing. Let me know if you want me to make any edits or add new tests, etc.

[timothycrosley]

This is great, thank you for fixing this!