Adjust the styles of your theme. the relevant CSS class is qa-tag-link. You can prefix the tag name with text like this (using the ::before pseudo element):
.qa-tag-link::before {
content: '#';
}
The color can be set via the background-color attribute:
.qa-tag-link {
background-color: orange;
}
and/or the border attribute:
.qa-tag-link {
border: 1px solid orange;
}
depending on whether or not you want a solid rectangle.
If you want the styles applied only on the homepage rather than every tag occurrence you need to further discriminate the selection:
.qa-template-qa .qa-tag-link {
...
}