This problem also relates to changes in the CURVE theme between RC3 and RC4.
SimplePortal blocks use h3.catbg, with a leftspan to get that curve in on the left. This is how the title block, category title, etc. was done in CURVE RC3.
But in CURVE RC4, it is done with cat_bar and h3.catbg.
For the base CURVE, and for the theme used here at SimplePortal, the change does not make any visible difference.
For Curve and the mehmene theme shown here, that curve in the upper left corner of catbg is created by making that part of the image the same color as the color underneath.
For your theme, there is a subtle difference. In the presence curve in the upper left corner of any catbg is created by making that part of the image transparant, so the color underneath shows through. Except, with the block titles styled the way they are, it is a different piece of background image that shows through this little oddly-shaped window. The difference is in main_block.png -- which was clearly designed to work with CURVE RC4, and would not look good with CURVE RC3.
SimplePortal's developers may not be able to adress this change immediately, and may not make it their highest priority, as it does not affect users of the default CURVE theme, whether they user CURVE RC3 or CURVE RC4. You should, however, be able to make a quick-and-dirty change in your blocks template.
WARNING: THE FOLLOWING SUGGESTION WILL ONLY POINT YOU IN THE RIGHT DIRECTION. IT IS NOT A TESTED SOLUTION. IT MAY NOT BE A VERY GOOD WAY TO HANDLE SimplePortal's CUSTOM BLOCK TITLE STYLES AND CLASSES.
In Portal.template.php, in
function template_block_curve.
<h3 class="', $block['style']['title']['class'], '"', !empty($block['style']['title']['style']) ? ' style="' . $block['style']['title']['style'] . '"' : '', '><span class="left"></span>';
Maybe something like this will work: Insert a line before the <h3 with
<div class="cat_bar">
And, where you have
</h3>';
you would need, to close the DIV.cat_bar you just opened:
</h3></div>';