>1) How do you set the font to Arial
That's not a question about XSLT, it's a question about your target vocabulary. If you're generating HTML, then find out how to set the font to Arial in HTML, and then write the XSLT to generate that HTML. If you're generating some other target vocabulary, such as XSL-FO or SVG, the same applies. You can't start writing XSLT code until you know what output you want to produce.
2) How do you make the <strong> tags work inside the RSS's <description> tag?
There aren't any tags between the description start and end tags. The angle brackets have been escaped: you should only do this when you want them to be treated as text rather than as markup. I don't know why people escape markup to make it look like text, it's bad practice but it's sometimes done. It makes it very hard to process in XSLT. One workaround that works in some environments - but not Firefox - is to use disable-output-escaping which causes the serializer to turn the text back into markup.
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference