First of all, really glad to see enhanced web development features coming, esp with LESS.
However, there appears to be something horribly wrong with LESS and the setting to show modified lines.
If you open a LESS file, then set All Languages to show Modified Lines, the syntax highlighting disappears, and SE starts to go a bit nuts and randomly marks different lines as modified when they were not.
Also, during my testing, the comment coloration for // lines in LESS files stops working for some reason - but unfortunately I cannot easily reproduce this behavior and am unsure what triggers it to break. Will post again if I find out ...or maybe its related to the above instability...