Hi Lara,
I'm Nik, an independent advisor here to assist you with this.
I have signed out and sign in and I too get the left panel colour white. It seems to be a new change in the latest update.
I have looked further but it seems to be a default theme and I don't see an option to change this behaviour.
Given the situation, If you have any suggestion on this, please send your feedback to the Microsoft product group by submitting your suggestion to Microsoft Teams UserVoice here:
https://microsoftteams.uservoice.com
Rest assured that Microsoft monitors and reviews each customer's feedback and ideas, some features would be added to the product and services based on customer feedback in the future. At the same time, Microsoft can’t guarantee any specific features or timelines but promise to read and consider all suggestions as feasible.
Thanks for your understanding.
Regards
Nik