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I know that there are some themes that use different specifications for individual blocks in all columns.
One example is the color_custom theme:
https://xoops.org/modules/library/singlefile.php?cid=1&lid=26
Others are the Penquin theme:
https://xoops.org/modules/library/singlefile.php?cid=1&lid=171
And the RM Forum theme, which doesn't seem to be in the Themes Library anymore.
What I would suggest is that you go to the
XOOPS Themes Library, take a look at the themes and see if you can find one or two that create the effect that you want. Then just download them from the main XOOPS site and take a look at the theme.html and style.css files to see how they accomplished it.