1. Customize the wording in the form appearing at the bottom of a post for people to subscribe to the blog

You can override the following language constants translation from the backend language override section.

COM_EB_SUBSCRIPTION_FORM_TITLE="Stay Informed"

COM_EB_SUBSCRIPTION_FORM_DESC="When you subscribe to the blog, we will send you an e-mail when there are new updates on the site so you wouldn't miss them."

COM_EASYBLOG_SUBSCRIBE_BLOG="Subscribe to the blog"

2. Customize the background image behind the text?

By default, we have light/dark style for this subscription form, you can configure this from the backend > Easyblog > setting > views > post (entry) > Subscription Form Style

If you would like to change the background image, the only way you can do is replace to your custom background image from this file JoomlaFolder/media/com_easyblog/images/bg-subscribe-form.svg