Note: Settings can take up to 24 hours to apply.
Summary
This setting will allow an organisation to customise the look and feel of the Teams app store (aka the Teams app catalogue) to a limited degree.

Settings
The customize store policy is located in the Teams admin center -> Teams apps -> Customize store. This is a global setting and applies to all users.

The following customisation settings are currently available:

Customise organisation logo	The logo should be 240×60. It should be no larger than 5 MB. Supported file formats: .svg .png .jpg
Customise small logo	The logomark should be 32×32 pixels. It should be no larger than 5 MB. The supported formats are: .svg .png .jpg
Customise the background colour	For choose a picture: The background should be 1212×100 pixels. It should be no larger than 5 MB. The supported formats are: .svg .png .jpg
Customize the text colour of your name	Select a colour using the MS colour picker.

What it looks like to an end user

Microsoft reference:
https://docs.microsoft.com/en-us/MicrosoftTeams/customize-your-app-store