I have this dark theme for the boards you might want to use.
If you don't already have it, install Stylus for Chrome, Firefox or Opera.
Click on the icon in the toolbar in the top right of your browser (for Firefox Android, Stylus is at the bottom of the three-dots-menu)
Use the 'Manage' button to the options page and click on Write new style. Name it whatever you please.
Insert the following code: