How to create custom issue that is triggered when CPU Steal Time > X% (Instead of default threshold)

We have a Built-in Rule that triggers at CPU Steal Time > 5% as shown below:

As per your needs, you might want to create a Custom Issue that is triggered when CPU Steal Time > X%. You can do that by clicking on the avatar icon in the upper right corner of the UI and select 'Settings'. Then expand "Knowledge Management" on the left and click on "Custom Rules" and then click on "Add New Rule". Select Host from the Entity Type Drop down and Steal (cpu.steal) from the Metric drop down. Configure your Time Window, Aggregation and Threshold (e.g. 85%) as you desire:

Then you have to create a Custom Issue that is based on above Custom Rule. You can also apply a filter to the Custom Issue if you only want it to apply to a subset of hosts instead of all hosts (e.g. using entity.zone):

Now, that gets you an Issue that will only trigger when CPU Steal > 85%, but it does not get rid of the existing built-in "CPU Steal Exceeds 5%" Rule. You should be able to do that by disabling that rule in the Settings->Knowledge Management->Built-In Rules screen: