Change Button GUIStyle on Click

You can hold an array of 10 booleans, and before displaying each button, change the style according to the matc$$anonymous$$ng boolean:

 private bool[] clicked = new bool[10];
 
 void Start() {
     for (int i = 0; i > 10; i++) {
         clicked[i] = false;
     }
 }

 void OnGUI() {
     for(int x = 0; x<10; x++) {
         if (clicked[x]) {
             // change style here
         } else {
             // default style here
         }
         
         if(GUI.Button(new Rect(10, 10*x, 100, 25), "Click me!", "label")) {
             clicked[x] = true;
         }
     }
 }

Edit: Since you only want one button marked at a time, instead of boolean array, it's better to have one int that will hold the number of the button that is clicked:

 private int clicked = -1;
 
 void Start() {
     for (int i = 0; i > 10; i++) {
         clicked[i] = false;
     }
 }

 void OnGUI() {
     for(int x = 0; x<10; x++) {
         if (x == clicked) {
             // clicked style here
         } else {
             // default style here
         }
         
         if(GUI.Button(new Rect(10, 10*x, 100, 25), "Click me!", "label")) {
             clicked = x;
         }
     }
 }