#available_prop_container,
#assigned_prop_container {
  display: flex;
  flex-direction: column;
  align-items: start;
}

.addprop-container {
  display: flex;
  align-items: center;
  justify-content: center;
}

.addprop-container label {
  display: flex;
  flex-direction: column;
}

.listbox {
  width: 250px;
  height: 300px;
  text-align: left;
}

.buttons {
  display: flex;
  flex-direction: column;
  align-items: center;
  margin: 0 20px;
}

.buttons button {
  margin-bottom: 10px;
  background-color: black;
  color: white;
  border-radius: 50px;
  font-size: larger;
  font-weight: bold;
}

<div class="addprop-container">
  <div id="available_prop_container">
    <label for="listbox1">Available Properties:</Label>
    <select name="all-properties" id="listbox1" class="listbox" multiple>
      <option value="1001">Test Line 1</option>
      <option value="1002">Test Line 2</option>
      <option value="1003">Test Line 3</option>
      <option value="1004">Test Line 4</option>
      <option value="1005">Test Line 5</option>
      <option value="1006">Test Line 6</option>
      <option value="1007">Test Line 7</option>
      <option value="1008">Test Line 8</option>
    </select>
  </div>

  <div class="buttons">
    <button type="button" class="prop-selection" onclick="moveAllOptions(listbox1,listbox2)"> >> </button>
    <button type="button" class="prop-selection" onclick="moveSelectedOptions(listbox1,listbox2)"> > </button>
    <button type="button" class="prop-selection" onclick="moveSelectedOptions(listbox2,listbox1)"> < </button>
    <button type="button" class="prop-selection" onclick="moveAllOptions(listbox2,listbox1)"> << </button>
  </div>
  <div id="assigned_prop_container">
    <label for="listbox2">Assigned to User:</Label>
    <select name="assigned-properties[]" id="listbox2" class="listbox" multiple>
    </select>
  </div>
</div>