Since build 2200 and the rework of CSS completions, the tab completion for custom variables (and already existing) do not work as expected.
color : #FFF;
color : #F /* now hit tab and "#FFF" doesn't show up */
replacing the "css_completions.pyc" in the css package with a earlier version and deleting "css_completions.py" seems to fix the issue. so there must be some changes to those files breaking the completition.
refs. github.com/danro/LESS-sublime/issues/9 for the sake of completeness.