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Bassman,
Since you are trying to create a simple fixed width theme, you will have to "hardcode" the width of each table. In simple words, you will have to define the exact width of every table inside your markup.
If you have done so and that one is the only page which gives you trouble, search for the respective template and see if there is any style rule attached there. For example, if the template reads something like:
<table class="foo" style="width: 100%">...
then remove the style property.
The problem you described above occurs when you define a width property and value inside your CSS. When you do so, the respective table "tends" to expand all the way to the edges of the screen.
Unfortunately, hardcoding the width values inside your theme.html is the only way to avoid this "issue".
Let me know how it went