I agree with CAsailer, a pair of "everyday" pants plus a nice blouse are formal enough to pass "formal standards".
There are no "dress police" in the dining room. Of course, jeans should not be considered for formal nights.
On one cruise to Taihiti , our luggage was lost for 9 of the 12 days. We had the blue jeans and t-shirts we arrived with. We learned to always pack a small suitcase with extra underwear as carry on. The ship offered to loan me some clothes for formal night, but a dress with sneakers, no make-up just did not feel right. So my husband and I both borrowed a Tux from the ship. Some people looked at me "funny", but........we did the best we could with what was offered. We survived! So did the other passengers!
The lesson to be learned from this is: Dress as you feel most comfortable. You should not care about the opinion of others. But of course, be respectful.