DEV: Raise exception for deprecations in core stylesheets (#31894)

lib/stylesheet/compiler.rb

@@ -63,6 +63,7 @@ module Stylesheet
           source_map_contents: true,
           load_paths: load_paths,
           silence_deprecations: %w[color-functions import global-builtin],
+          fatal_deprecations: options[:strict_deprecations] ? %w[mixed-decls] : [],
           quiet: ENV["QUIET_SASS_DEPRECATIONS"] == "1",
         )
 

lib/stylesheet/manager/builder.rb

@@ -48,6 +48,7 @@ class Stylesheet::Manager::Builder
           color_scheme_id: @color_scheme&.id,
           load_paths: load_paths,
           dark: @dark,
+          strict_deprecations: %i[desktop mobile admin wizard].include?(@target),
         )
       rescue SassC::SyntaxError, SassC::NotRenderedError, DiscourseJsProcessor::TranspileError => e
         if Stylesheet::Importer::THEME_TARGETS.include?(@target.to_s)