Disclaimer: This texture pack takes advantage of core shaders, and it is not possible to port it to 1.18. Please do not ask me for it. If you want a different block selection outline, then update your game.
This texture pack changes the colour of the outline that appears when you select a block.
Disclaimer 2: The outline is not as thick as it is in the icon. It was made thicker in the outline to be more visible, refer to the gallery to check how thick it actually is.
All Other Colours:
Purple Block Selection Outline