If your dashboard will be used in a dimly lit office, consider switching its theme from light to dark, which causes the dashboard to be more readable and result in less eye strain in dark environments. To do so, click Options in the top right corner of the screen, and under Theme click Dark.
If you have a map element in your dashboard, consider also making the basemap dark to match the theme. Note that if you change the basemap within the dashboard, your changes will not be saved. To permanently change the basemap, this must be done within the web map item itself in your ArcGIS organization. See Choose basemap for details.