I would very much appreciate the possibility to make toggl track look like it did before the interface change.
To my eyes the purple sidebar was immensely more aesthetic and I don’t see the harm in bringing it back optionally.
Friendly Regards 
Thanks for sharing your thoughts on the interface design. I understand you preferred the previous look with the purple sidebar. Unfortunately, the ability to switch back to the old theme or use the previous visual style is not currently available in Toggl Track.
The current version includes dark mode and light mode options, which are the theme choices available to customize your experience. While I can’t restore the old theme, these modes do allow you to adjust the appearance based on your preference.
That said, this is great feedback, and I appreciate you sharing it. Your preference for the previous design is valuable input that the product team will consider for future updates.