3rd party themes suddenly stopped working

Bug Reports Resolved
1

Description

my installed theme suddenly isn’t used with a recent update. it’s still in the packages folder.

To Reproduce…

Steps to reproduce the behavior:

Expected Behavior

Screenshots

Setup

  • OS and Version:
    • Installation Method:
  • Mailspring Version:

Additional Context

2

I did notice that yeah, this is how you can fix it:

  1. in the theme folder you’ll see package.json, in it you find name, change it to one word without any symbols

  2. and the folder name of the theme should also not contain any symbols, then reinstall your theme

3

This was caused by a package name validation change in v1.21.0 that was stricter to prevent path injection attacks — it rejected any package name containing uppercase letters, which broke some existing themes. This is fixed in v1.21.1 (released May 19, 2026). Please update and your themes should load again without any renaming needed.