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Many themes use fixed widths. Check the style.css file included in your current theme.
In this particular case, in your theme.html document, the main table is defined as having an id "okvir". If you look in the style.css file that comes with the theme.html you will see that "okvir" tables are set to display at a width of 758px. You can edit the css file and change it to whatever width you like. Say 100% to fill the screen width. Of course it may have a knock on effect and you may need to adjust a few other values in the css file to get the theme to display exactly as you want it.