Adding HP Omen theme

[jaherron]

Description

I added an image in the backgrounds folder for HP Omen, and ran the Actions workflow. An entry for this theme was also added in the themes.json file, but I don't know where it should have gone so I put it below the one for generic HP.

I wanted to make this contribution since I recently got an Omen laptop and I like using these themes for when I install distros that don't come with a GRUB theme.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project (correct file and folder structure)
• The background image for theme follows naming convention: theme_name.png
• I updated themes.json file (JSON database)