Hello,
A few days ago on the blog was announced that the Dressprow theme will no longer be available after version 3.4.6 in September 2019. Maintaining both themes has proven quite a challenge; every aspect of phpList’s interface must be tested for each release twice over, once for each theme, in order to catch issues — particularly those relating to JavaScript.
Three months ago, a poll of the most used phpList theme was posted on the forum, no respondents indicated they used Dressprow.
Although, Dressprow will be available for installation via GitHub! Here is the Dressprow repo, which means that if you are looking to contribute to phpList you can become the theme maintainer in the future! 
Cheers,
Mariana