Fix side titlebar color in alpha-black preset

[rstanuwijaya]

Update Preset: Alpha Black

Description

Updated the CSS to fix a color theme bug on nautilus sidebar header.

Screenshots (optional)

Before:

After:

Wallpaper

Checklist

Before submitting the PR, I checked:

• [x] I've only modified one preset
• [x] I've checked the format of each json files I modified
• [x] (optional) I've added screenshots
• [x] The preset name is following preset naming guidelines.