Working with the TreeListViewItem's / Cells from code is possible but is more difficult because cells can be recreated or recycled and it is difficult to manage the changes.
It is best to work with your data and databind the FontWeight property. If this property is observable, you will be able to change it easily at runtime as well. I modified one of the previous projects I sent you, it is attached.
The border that appears when you click the hyperlink button is the focus visual of the TreeListViewItem. It appears because the control that you click (the HyperlinkButton) cannot be focused. This is determined by the IsTabStop property. You can of course disable the focus of TreeListViewItems as well but this will mean that there will be no keyboard navigation within the control.
A better approach here may be to make the hyperlink focusable (IsTabStop=true) but edit its template to remove its border.
Likewise, the template of the TreeListViewItem can be edited to remove its border as well.
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