Hi @Kelly Chang,
Although you might not have access to the css file directly you can remove the italics by adding styles directly inline to the <p> element, or adding overriding styles to the <style> section that is located at the top of the template file. If you don't want to edit the template or don't have access to it you can change the style on a page by page basis by adding the overriding styles to the custom html section located in the editing window of each landing page. See my screen shots below:
This is the style you want to change:
I added "!important" to the font style to make sure it definitely takes priority over the italic style. It probably isnt required as page level styles will trump externally inherited styles but it is just a good way to be sure. I also think it is the padding that you will want to reduce rather than the line spacing but I haven't touched that in the example below.
<style>
#heroText {
position: relative;
font-size: 20px;
line-height: inherit;
font-style: normal !important;
font-weight: normal;
padding: 10px 0 25px;
}
</style>
Go to the Edit Page Meta Tags Section:

In Custom Head HTML section, paste the style I have added above.

Now preview your page and you should be golden

Let me know how you get on.
Thanks,
Gerard