I know that there is no such thing as a “theme” for any liturgical celebration. In every liturgy we celebrate the Paschal Mystery and every aspect of our salvation is present there. So, on Palm Sunday we celebrate everything. Nonetheless the Roman Missal informs us that it is Palm Sunday of the Passion of the Lord. So both the Entry into Jerusalem and the Passion are main foci of the celebration.
Personally, I have always felt that the Passion part has been given short shrift. We rarely speak about the Passion element of this Sunday. Yes, we read the Passion from the Synoptics as the Gospel of the Mass. But popular piety is usually concentrated on the much shorter Gospel at the start of the Mass when Jesus Triumphal Entry into Jerusalem is proclaimed. The Passion Gospel is almost a dry run for the regal Passion Gospel from John on Good Friday.
This year I, like many others, will celebrate the Sunday at the start of Holy Week without any of the normal pomp and circumstance. Here in the Republic of Ireland, we are going to celebrate Holy Week and the Triduum without an assembly in the churches. Processions are not encouraged this year. Even the Congregation for Divine Worship recognizes that in most places a simple entrance will suffice for the Eucharistic liturgy.
God willing, we will have all the liturgical hoopla back next year and struggle with the multivalence of the twin poles of Passion and Palms. But this year we shall look more intently at Christ who “as an example of humility for the human race to follow … [took] flesh and submit[ted] to the Cross, that we [might] heed his lesson of patient suffering and so merit a share in his Resurrection.” (Collect of the Palm Sunday of the Passion of the Lord from the Roman Missal)