Commented Unassigned: Table Theme [9499]

Attached is an excel file with a custom Table Style. The loading of the table theme borks because enum.parse can't find a value. Recommend changing the theme to a string and using enum.tryparse. Have a helper that lets you get standard excel themes but still allows custom theme names.
Comments: [Github pull request](https://github.com/ClosedXML/ClosedXML/pull/6).