feat(frontend): support dark mode

[nakasyou]

I added dark mode support. I have heard that darkmode is important for websites.

Notes

• To implement, I chose some colors such as background. It can be changed.
• I used some !important in CSS to overwrite markdown style.
• I have paid attention to it, but light mode may remain.
• I didn't implement a switch to change mode. You can change mode by changing OS theme.

Screenshots