A style can be applied to a report in order to change its appearance and characteristics. You can create and set up your own styles in JReport Designer. When you publish your reports to JReport Server, you can include these custom styles with the published reports. When you run a report, the style feature will be enabled and you can select a style to apply to the report. After applying a style, the banded objects, crosstabs, charts, and tables in the report will take a uniform appearance.
By default, the style feature is enabled in JReport Viewer. In order to apply a style to a report, click Menu > Report > Style and select the required one from the submenu, or select the required style from the style drop-down list 
on the toolbar. You can also right-click a table, crosstab, chart, or banded object in the report and select Apply Style from the shortcut menu to select the required style in the Apply Style dialog. However, if there is only one style available to the report, this style will be applied to the report by default, in which case, you will find that all these style related commands are hidden.
Reference: See also the chapter Styles in the JReport Designer User's Guide for more information about how to work with styles at report design time.