Block passing --activate in theme install with multiple themes

[ernilambar]

Bug Report

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• [x] Yes, more specifically, I reviewed the guidelines on how to write clear bug reports.

Describe the current, buggy behavior

When two themes are passed to install with activate flag, first theme is activated and after second theme is installed, newly installed theme is activated.

wp theme install simple-life blue-planet --activate

May be we should warn users in such case like:

Error: Only a single theme can be active.

Describe what you would expect as the correct outcome

Error: Only a single theme can be active.

[danielbachhuber]

Good catch!

Another option is a warning instead of an error. I think I'd opt for the warning because throwing an error would be a breaking change.