the_excerpt tag

When using the gallery function in the image upload I am having some problems with one of my themes that uses the excerpt tag.
When there is a gallery added to a post and the use of the_excerpt() the following is displayed

gallery {
margin: auto;
}
.gallery div {
float: left;
margin-top: 10px;
text-align: center;
width: 33%; }
.gallery img {
border: 2px solid #cfcfcf;
}

I have also tried using the_excerpt_reloaded plugin, where the following error is displayed
[gallery]

css not displayed but still in error.

Anythoughts?