How can I center the text in the headers for an AG-grid control?

You may use this property:

cellStyle: {textAlign: 'center'}

I'm using react and I just added the following snippet to my Table component's .css

.ag-header-cell-label {
   justify-content: center;
}

I'm overriding the .css class class from ag-grid which I found via devtools inspection, and discovered it's using flexbox for display.

See example (not react but same concept): https://embed.plnkr.co/O3NI4WgHxkFiJCyacn0r/