delete themes and replace horizontal line styles
1. deleted the alternate theme directories which will be added back in in branches
2. In readme.md, added in the note for using `-` and for using horizontal lines under headings
3. In userDefineLang.xml, added .markdown as an extension, changed the horizontal lines to be prefixed keywords that then keep their style for 3+ dashes, e.g. `---...` or `===...`