diff --git a/src/pyj/dom.pyj b/src/pyj/dom.pyj
index 78ea0a975c..e95cf8208d 100644
--- a/src/pyj/dom.pyj
+++ b/src/pyj/dom.pyj
@@ -133,3 +133,12 @@ def add_extra_css(func):
 
 def get_widget_css():
     return extra_css.join('\n')
+
+
+def set_radio_group_value(parent, name, val):
+    changed = False
+    for inp in parent.querySelectorAll(f'input[name={name}]'):
+        inp.checked = inp.value is val
+        changed = True
+    if not changed:
+        raise KeyError(f'No radio group with name={name} found')
diff --git a/src/pyj/read_book/prefs/colors.pyj b/src/pyj/read_book/prefs/colors.pyj
index eb996b0089..f3a922ebaa 100644
--- a/src/pyj/read_book/prefs/colors.pyj
+++ b/src/pyj/read_book/prefs/colors.pyj
@@ -6,7 +6,10 @@ from elementmaker import E
 from gettext import gettext as _
 
 from book_list.globals import get_session_data
-from dom import add_extra_css, build_rule, clear, set_css, svgicon, unique_id
+from dom import (
+    add_extra_css, build_rule, clear, set_css, set_radio_group_value, svgicon,
+    unique_id
+)
 from modals import error_dialog
 from read_book.globals import default_color_schemes, ui_operations
 from read_book.prefs.utils import create_button_box
@@ -39,11 +42,6 @@ def get_container():
     return document.getElementById(CONTAINER)
 
 
-def set_radio_group_value(parent, name, val):
-    for inp in parent.querySelector(f'input[name={name}]'):
-        inp.checked = inp.value is val
-
-
 def resolve_color_scheme(current_color_scheme):
     sd = get_session_data()
     cs = current_color_scheme or sd.get('current_color_scheme') or defaults.current_color_scheme
@@ -93,7 +91,7 @@ def edit_color_scheme(ev):
         for which in MARGINS:
             attr = f'margin_{which}'
             val = scheme[attr]
-            set_radio_group_value(container, f'input[name={attr}_color_type]', 'custom' if val else 'default')
+            set_radio_group_value(container, f'{attr}_color_type', 'custom' if val else 'default')
             if val:
                 bg, fg = val.split(':')
                 container.querySelector(f'input[name={attr}_bg]').value = bg