Many people consider joy to be the theme of Philippians.
I would argue that at best joy is a subtheme of the letter, which is all about "gospel partnership." Take just chapter 1 as an example:
1:4-5: Here Paul rejoices specifically because the Philippians had become partners with him in gospel ministry. "I rejoice because of your participation in the gospel."
1:16. Paul rejoices because "Christ is being proclaimed."
1:18: Paul continues to rejoice because he knows that through their prayers and the help of the Holy Spirit this would turn out for his deliverance from prison. This would give him further opportunities to preach Christ.
Joy is never the goal.
Joy is always a byproduct.
Joy is the byproduct of living to spread the saving message of the gospel.