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A style (or custom template edits) can cause an error because the templates do not have the standard code expected by template modifications used by the group system or any other add-on that might need to change a section of a template. XenForo 2.x is not forgiving of this like XF 1.x was. If the code in a template isn't right, it will throw that error. In the case of Ui.X, the PAGE_CONTAINER template is a totally custom template and the add-on requires that template to be the exact one supplied by ThemeHouse. If it's an earlier version, or if it's been edited by you the error you're getting will more than likely happen.
Most likely, the exact section in the Ui.X Dark PAGE_CONTAINER template that's causing the error is this:
Code:
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If it does not contain that exact code, then you aren't running Ui.X 2.0.0.2 or you've edited that section and the error will happen.
Custom template edits should never be done to the templates themselves. They should be done with the template modification system where an execution order can be assigned, and they can be disabled or adjusted if needed.
First make sure you're using Dark 2.0.0.2 if you just downloaded and installed it. It is possible you installed 2.0.0.0 or earlier. Go to Admin->Appearance->ThemeHouse styles and make sure the left button for the dark style says "Force reinstall" and does not say "Upgrade". If it does say "Upgrade", then you don't have 2.0.0.2 installed.
Second disable all other add-ons and try installing again.
If you still get an error, and any custom template changes you made involve the PAGE_CONTAINER template, revert those changes. And try installing again.