Theme is most important part of odoo website and its is also use to make odoo website.
When the themes have been installed only after then XML files loaded, and whenever you make changes on an xml file then each and ever time you will have to force reloading.

for example see below code

<!-- Customize header  -->
<template id="custom_header" inherit_id="website.layout" name="Custom Header">

  <!-- Assign an id  -->
  <xpath expr="//div[@id='wrapwrap']/header" position="attributes">
    <attribute name="id">my_header</attribute>
  </xpath>

  <!-- Add an element after the top menu  -->
  <xpath expr="//div[@id='wrapwrap']/header/div" position="after">
    <div class="container">
      <div class="alert alert-info mt16" role="alert">
        <strong>Welcome</strong> in our website!
      </div>
    </div>
  </xpath>
</template>