@Morganga It’s not working because width is calculated dynamically and affecting the margin. You can changed the width offset to adjust it using custom.css. See below.
body
{
width: calc(100% - 250px);
}
Adjust the value of 250px for your need.
Before
After
@shalumov Powering off the raspberry pi directly from power strip is not recommended as it might corrupt the SD card. Your monitor is not switching on after powering on but stays in stand by can be solved using Raspberry pi itself. check Remote-control or other modules/scripts in this forum who supports it. If you monitor support CEC or other standard then it is doable with raspberry pi and you do not need to use Monitor/TV remote or button.
@Bolovai you can use slim power strip to power the display and pi and only one cable needs to be managed. you can also manually switch off both of them after shutting down pi (optional) from the power strip button or cable.