Is altering theme settings during a UI draw allowed?

This comes from a question on BSE where someone was altering a theme setting in a method that was added to the TOPBAR_HT_upper_bar class.

When this appended method is altering a theme color, the 3Dviewport gizmos fail to work properly, while the question reports gizmo highlighting fails and drags are delayed, I found the gizmos didn’t respond at all.

Is altering a theme setting (or any preferences/data) allowed during a UI draw? Can this be considered a bug? Is there something that can be done to prevent this or can warning messages be generated? Something like when an addon accesses data during registration. Does appending a method circumvent existing preventions?

Issue creating code :-

def TEST_HT_header_color_change(self, context):
    if bpy.context.scene.tool_settings.use_keyframe_insert_auto == True:
        bpy.context.preferences.themes[0] = (0.4, 0.000000, 0.000000, 1.000000)

        bpy.context.preferences.themes[0] = (0.137255, 0.137255, 0.137255, 1.000000)

def register():

def unregister():