You can apply styling changes to a report to help it visually matching the styling of your other content and make it easier
to read and more visually appealing. You can style a report from the Style tab 
.
On the Style tab, you can select a theme (.sty file, or StyleSheet), to apply to your report. Themes style multiple components of a report, such as headers, column titles, and data text, all at once. Themes include associated cascading style sheet files along with a StyleSheet to help coordinate styling between content items and pages into which they can be placed.
You can use a default theme, such as the Designer 2018, Light, or Midnight theme, or you can click Custom to select a legacy template or a StyleSheet saved to a domain or folder in your Repository. When you select a StyleSheet using the Custom option, it is applied to the report and added to the Theme list.
To make a custom theme available directly from the Theme menu, create a folder for it in the Global Resources domain, in the Themes folder. In that folder, you can add a custom StyleSheet to use with charts and reports, and a custom CSS file to use for pages. The .sty file and .css file must both be named themes.
Tip: When creating a custom theme, you can copy the CSS and StyleSheet syntax from existing themes to use as a model.