widget in group inserts HTML content into the group. The HTML can be any valid HTML - it doesn't just insert CSS styles.
If the template contained just the text:
background-image: url("/Pictures/metal_2.jpg");
then you would end up with that text on the dashboard - because it is just that, a piece of text.
If you want the page to know this is intended as a CSS style, then it needs wrapping in a <style> tag and it needs to identify what element on the page the rule is for.
Hence why we have said repeatedly that the template should be:
<style>
#TEST_Home{
background-image: url("/Pictures/metal_2.jpg");
}
</style>
The <style> tag applies to the whole page. That is just how it works. That is not a Dashboard thing - it is a basic rule of how the style tag works in HTML.
It is perhaps best you had a break from this. I have explained this multiple times. I have run out of ways to try to explain it.
