For some reason CSS selectors aren't highlighted when Sunburst theme is selected (as well as some other themes).
I've looked trough theme file and after I've disabled following ruleset (line 335)
<string>source entity.name.tag, source entity.other.attribute-name, meta.tag.inline, meta.tag.inline entity</string>
everything became all right and selectors was highlighted.
I assume that there is some bug in rulesets priorities or weights, since this rule was overriding more specific CSS selector rules.