chore: Replace darkmode mixin with useDarkMode composable [CW-3474] (#9949)

# Pull Request Template

## Description

Replaces darkModeMixin with the new useDarkMode composable and replaces
wll usages of mixin the the composable in components and pages

Fixes
https://linear.app/chatwoot/issue/CW-3474/rewrite-darkmodemixin-mixin-to-a-composable

## Type of change

Please delete options that are not relevant.

- [x] New feature (non-breaking change which adds functionality)

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>
Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>

app/javascript/widget/components/PreChat/Form.vue

@@ -7,7 +7,7 @@ import { isEmptyObject } from 'widget/helpers/utils';
 import { getRegexp } from 'shared/helpers/Validators';
 import { useMessageFormatter } from 'shared/composables/useMessageFormatter';
 import routerMixin from 'widget/mixins/routerMixin';
-import darkModeMixin from 'widget/mixins/darkModeMixin';
+import { useDarkMode } from 'widget/composables/useDarkMode';
 import configMixin from 'widget/mixins/configMixin';
 
 export default {
@@ -15,7 +15,7 @@ export default {
     CustomButton,
     Spinner,
   },
-  mixins: [routerMixin, darkModeMixin, configMixin],
+  mixins: [routerMixin, configMixin],
   props: {
     options: {
       type: Object,
@@ -24,9 +24,8 @@ export default {
   },
   setup() {
     const { formatMessage } = useMessageFormatter();
-    return {
-      formatMessage,
-    };
+    const { getThemeClass } = useDarkMode();
+    return { formatMessage, getThemeClass };
   },
   data() {
     return {
@@ -131,25 +130,28 @@ export default {
       return `mt-1 border rounded w-full py-2 px-3 text-slate-700 outline-none`;
     },
     isInputDarkOrLightMode() {
-      return `${this.$dm('bg-white', 'dark:bg-slate-600')} ${this.$dm(
-        'text-slate-700',
-        'dark:text-slate-50'
-      )}`;
+      return `${this.getThemeClass(
+        'bg-white',
+        'dark:bg-slate-600'
+      )} ${this.getThemeClass('text-slate-700', 'dark:text-slate-50')}`;
     },
     inputBorderColor() {
-      return `${this.$dm('border-black-200', 'dark:border-black-500')}`;
+      return `${this.getThemeClass(
+        'border-black-200',
+        'dark:border-black-500'
+      )}`;
     },
   },
   methods: {
     labelClass(context) {
       const { hasErrors } = context;
       if (!hasErrors) {
-        return `text-xs font-medium ${this.$dm(
+        return `text-xs font-medium ${this.getThemeClass(
           'text-black-800',
           'dark:text-slate-50'
         )}`;
       }
-      return `text-xs font-medium ${this.$dm(
+      return `text-xs font-medium ${this.getThemeClass(
         'text-red-400',
         'dark:text-red-400'
       )}`;
@@ -264,7 +266,7 @@ export default {
       v-if="shouldShowHeaderMessage"
       v-dompurify-html="formatMessage(headerMessage, false)"
       class="mb-4 text-sm leading-5 pre-chat-header-message"
-      :class="$dm('text-black-800', 'dark:text-slate-50')"
+      :class="getThemeClass('text-black-800', 'dark:text-slate-50')"
     />
     <FormulateInput
       v-for="item in enabledPreChatFields"