How to change the style of a div?

Question

css

Hello. I can't make changes to the created styles on a site created with joomla.
I seem to be doing everything right, but nothing comes out.
In php file I frame the code with a tag

<div class="search-box"><h3><?php echo $this->keyword; ?></h3>

  <form action="<?php echo JRoute::_ ('index.php?option=com_virtuemart&view=category&limitstart=0', FALSE); ?>" method="get">

    <!--BEGIN Search Box -->
    <div class="virtuemart_search">
      <?php echo $this->searchcustom ?>
      <br>
      <?php echo $this->searchCustomValues ?>
      <input name="keyword" class="inputbox" type="text" size="20" value="<?php echo $this->keyword ?>"/>
      <input type="submit" value="<?php echo vmText::_ ('COM_VIRTUEMART_SEARCH') ?>" class="button" onclick="this.form.keyword.focus();"/>
    </div>
    <input type="hidden" name="search" value="true"/>
    <input type="hidden" name="view" value="category"/>
    <input type="hidden" name="option" value="com_virtuemart"/>
    <input type="hidden" name="virtuemart_category_id" value="<?php echo $category_id; ?>"/>

  </form>
  <!-- End Search Box --></div>

There is no such class anywhere else in the project.
Then in the theme file I write

.search-box {
    background-color: black;
}

But nothing changes.
Site allstroi

Answer 1

Do not forget to update the cache!!!)))))) Thank you all!

Answer 2

Should frame like this:
And check the height of the element, if it's 0px, then you won't notice the backgrounds.

Answer 3

Open the site in Chrome -> Hover over the desired element -> Open the context menu (right mouse button) "View element code" -> In the right part of the window that opens, look where the styles come from and why yours do not work.
There you will also see if your frame is displayed ...