Avoid TypeError in getCSSVar when theme is an empty object

Editor assessment

The subject states the defect and its trigger exactly: getCSSVar avoided a TypeError when the theme is an empty object. The utils scope is a proper package noun, the fix type matches a crash correction, and both the function and the failing condition are named, which is the complete failure story. There is no body, acceptable since the headline carries the exact guard scenario. No footers appear. The lowercase imperative phrasing is clean. Compact, precise, and self-sufficient without the diff.