Add color options for background in main.html

[sunho]

Add a theme color options.

[codacy-production[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
:white_check_mark: +0.00% (target: -1.00%)	:white_check_mark: ∅

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (17c5522e0712144bc00ddcc3a6304a19e2a7b21c) | 496 | 287 | 57.86% | | | Head commit (10a6e4c81fc3a76590959ab809d7aa7a53079c4c) | 496 (+0) | 287 (+0) | 57.86% (**+0.00%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#107) | 0 | 0 | **∅ (not applicable)** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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