fix: 修复auto、dark模式下在刷新后，code模块未同步的情况

[BARMPlus]

Description

#226
auto模式下，依然会出现code模糊的问题，修复此问题。
react-syntax-highlighter style属性直接设置为dark或者light常量，可以正常生效，但是用useMemo或者其他方式包裹后返回的变量，始终无法完成正常的初始化。
解决方案：mode初始值设置为undefined，根据theme重新设置。

Types of changes

• 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 change)
• Enhancement (changes that improvement of current feature or performance)
• Refactoring (changes that neither fixes a bug nor adds a feature)
• Test Case (changes that add missing tests or correct existing tests)
• Code style optimization (changes that do not affect the meaning of the code)
• Docs (changes that only update documentation)
• Chore (changes that don't modify src or test files)