I ran into another issue.
This change introduces $danger-low-mid in 2.6.0beta2.
This broke the discourse-styleguide plugin so that was updated and a .discourse-compatibility file was introduced to keep the plugin at the previous commit for Discourse 2.5.0.
This breaks on Discourse 2.5.1. Since this change will never be backported to stable, the discourse-compatibility file would need to be updated on every new 2.5.x stable version.
Every discourse-compatibility file of every plugin would need to be updated on every new 2.5.x stable version.
Alternatively, the compatibility file could refer to version 2.5.999, effectively keeping the plugin at commit 1f86468b2c81b40e97f1bcd16ea4bb780634e2c7 for the entire 2.5.x lifetime. But this seems very hacky to me.