Add Theme Guideline

[jjnether]

Added guideline for no themes from opening sequences

Description

Added a guideline for themes to not be from opening sequences.

Screenshot

Issues Fixed or Closed

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Dependency update (updates to dependencies)
• [x] Documentation update (changes to documentation)
• [ ] Repository update (changes to repository files, e.g. .github/...)

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch must be updated before it can be merged. You must also Allow edits from maintainers.

• [ ] I want maintainers to keep my branch updated

[codecov[bot]]

Codecov Report

Merging #2036 (26e7bce) into master (ceab515) will decrease coverage by 67.20%. Report is 14 commits behind head on master. The diff coverage is 25.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2036 +/- ## =========================================== - Coverage 85.98% 18.78% -67.20% =========================================== Files 1 1 Lines 314 314 =========================================== - Hits 270 59 -211 - Misses 44 255 +211 ``` | [Files](https://app.codecov.io/gh/LizardByte/ThemerrDB/pull/2036?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LizardByte) | Coverage Δ | | |---|---|---| | [src/updater.py](https://app.codecov.io/gh/LizardByte/ThemerrDB/pull/2036?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LizardByte#diff-c3JjL3VwZGF0ZXIucHk=) | `18.78% <25.00%> (-67.20%)` | :arrow_down: |