Removing default theme no longer in use

docker/dev/config_docker.json

@@ -11,7 +11,6 @@
     "TeamSettings": {
         "SiteName": "Mattermost",
         "MaxUsersPerTeam": 50,
-        "DefaultThemeColor": "#2389D7",
         "EnableTeamCreation": true,
         "EnableUserCreation": true,
         "RestrictCreationToDomains": ""