I tried this and it worked fine for me. So I would suggest:
1) You're modifying the wrong .tmTheme file. The one referred to from Preferences> Color Scheme > Color Scheme - Default is in Packages/Color Scheme - Default (on my Windows).
2) You've made a typo modifying the XML so that it is malformed:
3) The scope is not 'keyword.operator' for the language you are using; try opening different language files. (This is an unlikely scenario.) Pressing Ctrl-Alt-Shift-P will also reveal the current scope in the status bar.
Option 1) seems the most likely.