In your version,there is no verb...
"May the flood of cheap dark beer............"
If you want to say that the beer is flowing,you must place the verb "llifo" (to flood) after "cwrw". The word "llif" is a noun - a flood.Therefore,"llif cwrw tywyll...." means "a flood of dark beer"....There is no verb in your sentence.You wrote "May the flood of dark cheap beer".Full stop.No verb.My apologies for being pedantic,but correct grammar is important,whatever the language.Nought out of ten for Google .