Duo accepts du café et du thé and café et thé as of 1 December 2015.
After researching, I found two possible grammar explanations for Kaffee und Tee without articles:
German (and English) do not use an article to express the partitive some: e.g. [some] bread, [some] wool. French, however, uses the partitive du/de la: e.g. du pain, de la laine.
German omits the article when two or more nouns, connected by und, express one idea.