feat(theme): Add lilac_mist (sandyg6)

[sandyg6]

Description

• Added a new color palette feature to themes.

Checks

• [ ] Adding quotes?
  • [ ] Make sure to include translations for the quotes in the description (or another comment) so we can verify their content.
• [x] Adding a language or a theme?
  • [ ] If is a language, did you edit _list.json, _groups.json and add languages.json?
  • [x] If is a theme, did you add the theme.css?
    • Also please add a screenshot of the theme, it would be extra awesome if you do so!
• [ ] Check if any open issues are related to this PR; if so, be sure to tag them below.
• [x] Make sure the PR title follows the Conventional Commits standard. (https://www.conventionalcommits.org for more info)
• [x] Make sure to include your GitHub username inside parentheses at the end of the PR title

Closes #