Channel switcher UI improvements (#6564)

* Channel switcher UI improvements

* Reverting mattermost theme

* Fixing spacing above the button

* Updating switcher button

* PLT-6691 - Fixing tablet header

webapp/utils/constants.jsx

@@ -481,28 +481,28 @@ export const Constants = {
         },
         mattermost: {
             type: 'Mattermost',
-            sidebarBg: '#1a1d1f',
-            sidebarText: '#ffffff',
-            sidebarUnreadText: '#e8e8e8',
-            sidebarTextHoverBg: '#525252',
-            sidebarTextActiveBorder: '#27a1b2',
-            sidebarTextActiveColor: '#ffffff',
-            sidebarHeaderBg: '#1a1d1f',
+            sidebarBg: '#fafafa',
+            sidebarText: '#333333',
+            sidebarUnreadText: '#333333',
+            sidebarTextHoverBg: '#e6f2fa',
+            sidebarTextActiveBorder: '#378FD2',
+            sidebarTextActiveColor: '#111111',
+            sidebarHeaderBg: '#3481B9',
             sidebarHeaderTextColor: '#ffffff',
-            onlineIndicator: '#1fc1d9',
-            awayIndicator: '#c4c492',
-            mentionBj: '#ba3939',
+            onlineIndicator: '#7DBE00',
+            awayIndicator: '#DCBD4E',
+            mentionBj: '#2389d7',
             mentionColor: '#ffffff',
             centerChannelBg: '#ffffff',
-            centerChannelColor: '#2d3138',
-            newMessageSeparator: '#1c8c99',
-            linkColor: '#1c8c99',
-            buttonBg: '#27a1b2',
+            centerChannelColor: '#333333',
+            newMessageSeparator: '#FF8800',
+            linkColor: '#2389d7',
+            buttonBg: '#23A2FF',
             buttonColor: '#FFFFFF',
-            errorTextColor: '#cc2d2d',
-            mentionHighlightBg: '#1c8c99',
-            mentionHighlightLink: '#ffffff',
-            codeTheme: 'monokai',
+            errorTextColor: '#a94442',
+            mentionHighlightBg: '#f3e197',
+            mentionHighlightLink: '#2f81b7',
+            codeTheme: 'github',
             image: mattermostThemeImage
         },
         mattermostDark: {