I guess I should provide some more details
There are 6 new keys in the theme:
xxxForeground controls the color of the underline and text, while xxxOptions contains a space separated list of flags, which may be 'underline' and 'foreground'. 'underline' will show an underline, while 'foreground' will show the text itself in a different color.
'bracketsXXX' is used when the caret is next to the brackets, while 'bracketContentsXXX' is used when the caret is between brackets. If you set all the bracketsXXX and bracketContentsXXX keys to the same value, then there will be no visual difference between having the caret next to a bracket or not.