Clicking on Set Default from Silver to Default or from Default to Silver in the Layout Settings >> Theme Settings it doesn’t changes from Silver to Default or from Default to Silver.
The solution is to locate the browser cookie specific to the website address (IP or hostname) and remove them every time ‘Set as Default’ is changed. Alternatively, a quicker way for theme change is to use the dropdown Theme option at the upper right corner on screen after login. Whenever theme is required to be changed, click on Set Default link and then switch between themes from dropdown Theme option at the upper right corner.