How use Common Template editor

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a. I did not add them directly to the template, but I used this form: admincp > style manager > thems > go to "common template". https://www.vbulletin.com/docs/html/...mmon_templates.
i add there in "header" field.
if the right way it edit directly the template, I would love to know what and how, for apply globaly all styles at onc.

b. not only if I write the code I brought brings an error.
But even if I just write "123". This is indeed written on the page, but the whole page goes wrong as if it lacks a critical closing tag.