Add a option for setting color contrast in dark mode

[serifold]

Hello, thanks for your contributing and sharing, I enjoy using this app very much.

However, today I updated V2er, then I found the colour contrast between text and background in dark mode is too sharp for me. In my opinion, the old version's colour contrast is comfortable; if the text is as white as the current version, my eyes get tired and a bit stung quickly.

So, could you give us a option, e.g. 'Enhance the colour contrast between text and background in dark mode', that we can choose whether turn it on or not according to ourselves' feeling? Thank you.

[Chernsha]

新版暗黑模式的對比度，我反倒感覺看起來非常舒適。不過多個選項切換新舊版的對比度，也許可以滿足不同人的需求。