The iXBRL Viewer toolbar is not visible by default. It must be enabled in the iXBRL Viewer settings. Then you will find the toolbar above the iXBRL Viewer. It contains some functions to make XHTML tables visible within a report.
The Highlight Tables button highlights XHTML tables within text block tags with a colored border. The color of the highlighting can be changed in the Settings in the main menu of the application. The Border Thickness settings control the thickness of this border. An adjustment may be necessary if the loaded report has been scaled down for optimal display and thin borders are then difficult to recognize.