I have a lot of themes in my test wordpress site. I get a little frustrated that I can't show more than 15 at a time.

In pages and posts you can click "screen options" to choose the number, but it looks like "Themes" has no such option.

Is there another way to increase the number?

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It's hardcoded into the WP_Themes_List_Table class that's responsible on Line 50.
\$per_page = 15;