I think that I understand what you want to do. Simply stated, you want to use the theme surrounding your Web Part except the gradient image.
The beauty of CSS (Cascading Style Sheet) is that it cascades. So, you will want to specify the built-in class attribute so that your Web Part will adopt the theme style except you override the gradient using the style attribute.
I copied the ows.css file and the partgrad.gif into the same directory as an HTM file containing the following code and then modified the ms-WPHeader definition in the CSS file to point at the copy of partgrad.gif so that I could test all of this locally. What follows is some code that demonstrates what I mean using the style that you want to implement. The following image demonstrates the result:
------------------HTML---------------
<html>
<Link REL="stylesheet" Type="text/css" HREF="ows.css">
<head>
<title>Override Style</title>
</head>
<body>
<table border="1" width="100%">
<tr class="ms-WPHeader">
<td id="WebPartTitleWPQ1" style="width:100%;">
<div class="ms-WPTitle">
<nobr>
<span>Using built-in class style
</span>
<span id="WebPartCaptionWPQ2">
</span>
</nobr>
</div>
</td>
</tr>
<tr class="ms-WPHeader">
<td id="WebPartTitleWPQ2"
style="width:100%;
Border-Right: black 0px solid;
Border-Left: black 0px solid;
Border-Top: black 0px solid;
Border-Bottom: black 0px solid;
Autogrow: true;
Filter: PROGID:DXIMAGETRANSFORM.MICROSOFT.GRADIENT(GRADIENTTYPE=1,
STARTCOLORSTR='#FFFFFF',ENDCOLORSTR='#D1DCF8')"
>
<div class="ms-WPTitle">
<nobr>
<span>Override built-in class style
</span>
<span id="WebPartCaptionWPQ2">
</span>
</nobr>
</div>
</td>
</tr>
</table>
</body>
</html>
----------------END HTML-------------