The search form input is found in index.html and searchrooms.html

The components are made of date picker, select groups and button.

<form action="searchresult.html">
...
    <div class="hg-form-component-input form-group">
     ...
   </div>
...
</form>

To change the options from select input just change the values of the following code:

<select id="reservation-people" name="adults">
  <optgroup label="Adults">
   <option value="0">Adults</option>
   <option value="1">1</option>
   <option value="2">2</option>
   <option value="3">3</option>
   <option value="4">4</option>
  </optgroup>
</select>

In order to change the styles of the search form, the "_searchForm.scss" and "_pages.scss" has to be modified.

The style locations are in app/style/scss/elements/_searchForm.scss and app/style/scss/_pages.scss