diff --git a/crates/ui-egui/src/theme.rs b/crates/ui-egui/src/theme.rs
index 66c3380..c667e6f 100644
--- a/crates/ui-egui/src/theme.rs
+++ b/crates/ui-egui/src/theme.rs
@@ -6,6 +6,13 @@ use std::sync::atomic::{AtomicBool, Ordering};
 /// Whether the Light palette is active. Dark is the default (the incumbent is dark-only).
 static LIGHT_THEME: AtomicBool = AtomicBool::new(false);
 
+/// Unit tests that change the process-wide palette must own it until their assertions finish.
+#[cfg(test)]
+pub(crate) fn lock_test_palette() -> std::sync::MutexGuard<'static, ()> {
+    static PALETTE: std::sync::Mutex<()> = std::sync::Mutex::new(());
+    PALETTE.lock().unwrap_or_else(std::sync::PoisonError::into_inner)
+}
+
 /// The Appearance setting (`ui.theme {mode}`). Dark by default; System and Light are opt-in.
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
 #[serde(rename_all = "lowercase")]
@@ -443,7 +450,8 @@ mod tests {
 
     #[test]
     fn application_frames_follow_system_appearance_and_preserve_manual_palettes() {
-        // Keep all frame assertions in one test: custom-painted tokens share LIGHT_THEME.
+        // Mixer rendering tests also change the process-wide custom-painted palette.
+        let _palette = lock_test_palette();
         fn frame(app: &mut SoundApp, ctx: &egui::Context, system_theme: Option<egui::Theme>, light: bool) {
             let mode = app.ui.theme;
             let colors = |app: &SoundApp| {
diff --git a/crates/ui-egui/src/widgets.rs b/crates/ui-egui/src/widgets.rs
index d4eab05..1eca07c 100644
--- a/crates/ui-egui/src/widgets.rs
+++ b/crates/ui-egui/src/widgets.rs
@@ -396,6 +396,7 @@ mod selector_visual_regressions {
 
     #[test]
     fn capture_selector_visual_fixtures() {
+        let _palette = crate::theme::lock_test_palette();
         let Some(directory) = std::env::var_os("CRAFT_UI_VISUAL_FIXTURES").map(std::path::PathBuf::from) else { return };
         std::fs::create_dir_all(&directory).unwrap();
         for scale in [1.0, 1.5, 2.0] {
@@ -451,6 +452,7 @@ mod selector_visual_regressions {
 
     #[test]
     fn actual_mixer_keeps_every_active_automation_mode_legible() {
+        let _palette = crate::theme::lock_test_palette();
         for narrow in [false, true] {
             for mode in soundcraft_model::AutomationMode::ALL {
                 for scale in [1.0, 1.5, 2.0] {
@@ -491,6 +493,7 @@ mod selector_visual_regressions {
 
     #[test]
     fn capture_actual_mixer_visual_fixtures() {
+        let _palette = crate::theme::lock_test_palette();
         let Some(directory) = std::env::var_os("CRAFT_UI_VISUAL_FIXTURES").map(std::path::PathBuf::from) else { return };
         std::fs::create_dir_all(&directory).unwrap();
         for mode in soundcraft_model::AutomationMode::ALL {
@@ -544,6 +547,7 @@ mod selector_visual_regressions {
 
     #[test]
     fn compact_mixer_selectors_keep_labels_on_one_visible_line() {
+        let _palette = crate::theme::lock_test_palette();
         for width in [46.0, 74.0, 110.0] {
             for label in ["auto read", "no group", "A very long mixer group name"] {
                 for scale in [1.0, 1.5, 2.0] {