Out of the box, Microsoft Teams adopts a light theme that consists of predominately white backgrounds and coloured action buttons.
This has been the standardisation of apps for a long time. However, for those who suffer with glare, a white backgrounded app can be damaging to eyes and add increased strain on them. This can result in eye health problems in the future.
As part of Microsoft’s commitment to accessibility, Teams has three default themes to choose from; Default (Light), Dark and High Contrast.
To change the theme, click on Settings from the ellipsis (…) on the top menu bar
From the General settings tab, choose your theme

Once chosen the app will reload into the chosen theme.
Dark Theme

Personally, this is my favourite theme for day to day operation. The white text on a dark background easily stands out and is easy to read without having to adjust my vision or seating position. I also find this mode a lot less tiring throughout the day.
High Contrast Theme

High Contrast theme is predominately aimed at people with low vision or photosensitivity.
Theme Behaviours
It is important to note that the theme you choose is saved locally on the Teams client it was chosen. Should you access Teams from another PC or mobile app, you will need to set your desired theme on each device.