These are only shifts in the presentation of the website's theme and color. Clicking on the Link toggles between Day Mode and Night node for those who want a less bright screen.
The settings are stored locally to the browser. When you close the session the next time will have the preference for whichever you've chosen retained.
Day ModeThis is the Default look of the 5eTools website.
Night ModeThis is the low light appearance of the site.
NOTE: not all of the site features are completely converted, layout elements may have odd colored lines etc. - there is a new UI layout in process so such minor shifts are NOT being worked upon.
Did that not answer your question(s)?
if not please check the Known Issues page for further guidance, or leave a comment below to let others know (and help you).