My heart is overflowing with a good theme

This is a song of praising an individual with the imagery of that of a King and royalty. This individual being praised may be the Messiah. For example, Grace is poured on Him, and God has blessed Him forever. He also has glory and majesty like a king but perhaps more. He is also characterise by truth, humility, and righteousness; whereas a human king usually lacks humility.
The next part may be a give away when it says "Your throne, O God". He is anointed by God, love righteousness and hate wickedness. It also says "He is your Lord, worship Him" - the Israelites know quite well that they are only to worship God alone and never worship any mere human kings. So this king in this passage is more than a human king.
So this song or psalm could in fact be a prophetic song about the Messiah Yeshua who is Lord of Lords, and King of Kings.

Psalm 45
To the Chief Musician. Set to “The Lilies.”[a] A Contemplation[b] of the sons of Korah. A Song of Love.

 1 My heart is overflowing with a good theme;
         I recite my composition concerning the King;
         My tongue is the pen of a ready writer.
        
 2 You are fairer than the sons of men;
         Grace is poured upon Your lips;
         Therefore God has blessed You forever.
 3 Gird Your sword upon Your thigh, O Mighty One,
         With Your glory and Your majesty.
 4 And in Your majesty ride prosperously because of truth, humility, and righteousness;
         And Your right hand shall teach You awesome things.
 5 Your arrows are sharp in the heart of the King’s enemies;
         The peoples fall under You.
        
 6 Your throne, O God, is forever and ever;
         A scepter of righteousness is the scepter of Your kingdom.
 7 You love righteousness and hate wickedness;
         Therefore God, Your God, has anointed You
         With the oil of gladness more than Your companions.
 8 All Your garments are scented with myrrh and aloes and cassia,
         Out of the ivory palaces, by which they have made You glad.
 9 Kings’ daughters are among Your honorable women;
         At Your right hand stands the queen in gold from Ophir.
        
 10 Listen, O daughter,
         Consider and incline your ear;
         Forget your own people also, and your father’s house;
 11 So the King will greatly desire your beauty;
         Because He is your Lord, worship Him.
 12 And the daughter of Tyre will come with a gift;
         The rich among the people will seek your favor.
        
 13 The royal daughter is all glorious within the palace;
         Her clothing is woven with gold.
 14 She shall be brought to the King in robes of many colors;
         The virgins, her companions who follow her, shall be brought to You.
 15 With gladness and rejoicing they shall be brought;
         They shall enter the King’s palace.
        
 16 Instead of Your fathers shall be Your sons,
         Whom You shall make princes in all the earth.
 17 I will make Your name to be remembered in all generations;
         Therefore the people shall praise You forever and ever.