This problem seems to be caused by a Visual Studio bug. It incorrectly handles bold highlighting used by ReSharper for enum members and constants when using some fonts.

Possible workarounds are:

1) Change the font in the editor - some fonts do not suffer from this

problem, e.g. Courier New

2) Turn off bold attribute for constants: open Tools > Options > Environment > Fonts and Colors page, find "ReSharper Constant" item in the list and uncheck "Bold" checkbox.