fix(@vben/web-ele): the main color tone for switching between dark and light themes has been reset (vbenjs#6678)

Author: pangxie231

packages/effects/layouts/src/widgets/preferences/blocks/theme/builtin.vue

@@ -104,7 +104,7 @@ function selectColor() {
 
 watch(
   () => [modelValue.value, props.isDark] as [BuiltinThemeType, boolean],
-  ([themeType, isDark]) => {
+  ([themeType, isDark], [_, isDarkPrev]) => {
     const theme = builtinThemePresets.value.find(
       (item) => item.type === themeType,
     );
@@ -113,7 +113,9 @@ watch(
         ? theme.darkPrimaryColor || theme.primaryColor
         : theme.primaryColor;
 
-      themeColorPrimary.value = primaryColor || theme.color;
+      if (!(theme.type === 'custom' && isDark !== isDarkPrev)) {
+        themeColorPrimary.value = primaryColor || theme.color;
+      }
     }
   },
 );