This summer I'm in a Bible study going through Philippians. We established last week that the theme of the book is joy ("Rejoice in the Lord always, and again I say, rejoice!" (4:4)). The average English translation has about 12 uses of joy/joyful/rejoice altogether.
The Greek text is a little more complex. Most of the words we translate as "joy" are forms of the root χαρα, but three of them are καυχημα. Then I tried marking every appearance of χαρα or καυχημα, no matter how it was translated. Total: 25. This book is oozing with joy, but not always in the way we might expect.
Many of the instances of χαρα are translated as "grace", as in 1:2: "Grace to you and peace from God our Father...." The closing blessing is the same: "The grace of our Lord Jesus Christ be with you all" (4:23). If we were to be unconventional and translate χαρα as "joy," the book's "theme" would be built right into its opening and closing lines, not just sprinkled across the middle.
It makes sense for joy and grace to be closely related concepts. How else could we be joyful if not for God's grace? Here, they're two sides of the same coin. However, some of the forms of χαρα add a new dimension to our idea of what "joy" and "grace" look like. For instance, we partake of grace with Paul in his chains (1:7).
Later, Paul says this: "To you it has been granted on behalf of Christ, not only to believe in Him, but also to suffer for His sake" (1:29). "Granted" is a verb form of χαρα: these things have been graced to us. And, as Paul points out, it's not that hard to be joyful about this tough grace, because we're imitating Jesus, which means that our suffering has a purpose.
"Let this mind be in you which was also in Christ Jesus... He humbled Himself and became obedient to the point of death, even the death of the cross. Therefore God also has highly exalted Him and given Him the name which is above every name" (2:5, 8-9). "Given" is χαρα again. Lest we think grace is something like cough syrup, vaguely beneficial but also pretty unpleasant, Paul makes it clear that the same grace also means joy, because we look to our reward.
That's the joy that Philippians is all about: not just the stuff of happy-clappy praise choruses, but the certainty that our sufferings are always rooted in grace.