As an extreme example, if you find an old minhag to put a fir tree in your living room on Channuka, and you decide to adopt that minhag because you want to fit in with your goyish neighbors, you are still doing something wrong- even if there is some basis. Because of your motivation.”
That would be because the fir tree has specifically come to symbolize the goyishe holiday. I would not color hardboiled eggs for Pesach, either. But making a Friday night oneg shabbos on the occasion of the birth of either a son OR daughter, does not seem to be commensurate with either of those things.