A detailed appreciation of Bai Juyi's "Grass". What is the theme?

The first sentence of the poem "Li Li Yuan Grass" closely links the three words of the title "Ancient Yuan Cao", and uses the repeated word "Li Li" to describe the lushness of spring grass. The second sentence, "One year old and one glorious year," goes on to describe the law of the wild grasses on the original land that dries up in autumn and flourishes in spring, and cycles year after year, with endless growth. In the third and fourth sentences, "Wildfire never burns out, but the spring breeze blows and grows again." One sentence is written as "Withered" and the other is written as "Rong", which is a play on the meaning of the word "Withered and Rong". No matter how mercilessly the fire burns, as long as the spring breeze blows, green weeds are everywhere again, which vividly demonstrates the tenacious vitality of weeds.

In the fifth and sixth sentences, "The distant fragrance invades the ancient road, and the clear green meets the deserted city." The words "invasion" and "connection" are used to describe the spreading spring grass and the vast green field. The "ancient road" and "deserted city" are also highlighted. Go to the place your friend is about to experience. The last two sentences, "Send off the king and grandson again, full of love for farewell", point out the original intention of farewell. The endless luxuriant spring grass is used as a metaphor for the feeling of farewell that fills the heart and fills the wilderness. It truly achieves the blending of scenes and has endless charm.

On the one hand, this poem describes the tenacious spirit of weeds, and also uses the tenacity of weeds as a metaphor to express how long-lasting my feelings of parting are.