Steps to reproduce: Follow the steps below to reproduce the issue.

• Enter theme slug in non-lowercase when activating it (Examples: wp theme activate Dara, `wp theme
• Notice a success message shown Success: Switched to 'Dara' theme.
• Enter wp theme list to notice that no themes are active

➜  local wp theme list
+-----------------+----------+--------+---------+
| name            | status   | update | version |
+-----------------+----------+--------+---------+
| dara            | inactive | none   | 1.2.1   |
| twentyseventeen | inactive | none   | 1.7     |
+-----------------+----------+--------+---------+
➜  local

What is expected: Theme activation does not go through, and the success notice is not shown.

What happens instead: A success message is shown, but no theme is not activated.

Screencast indicating this behaviour: https://asciinema.org/a/JICHDMDwcPjsVibxZMUlHOHRJ

In theory, slug is supposed to be entered in lowercase.

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• Tested on a local install of WordPress. WordPress 4.9.8
• PHP version: 7.2.8
• WP-CLI version: 2.0.0