Changing a repeater box background color based on what URL source is in the video player

It is not as easy as you think, but of course it is possible.
You have to loop through all the repeater searching for the item that have the same source as the current displayed video. It would be something like this:

$w.onReady(() => {
  $w('#repeaterVideoItem').onClick(({ context }) => {
    let $item = $w.at(context)
    $w('#currentVideo').src = $item('#repeaterVideoItem').src

    $w('#repeater').forEachItem(($i) => {
      $i('#repeaterVideoItem').style.backgroundColor =
        $w('#currentVideo').src === $i('#repeaterVideoItem').src
          ? '#3637f5'
          : '#ffffff'
    })
  })
})

Remember to change the elements ids to the ones on your project.

PS: Not tested.