Enhanced Dark Mode

[KyleJKC]

Adjusted background and text color in dark mode according to issue #3472.

The new background color #161c23 is created under the instruction of Material Design 2 Guidelines, by overlaying the primary color #1d93ab with #121212 at a low capacity.

The new text color is pure white #ffffff according to the recommendation of the guidelines.

The new contrast ratio of background and text is now 17.15:1.

Also tweaked the hover-color a bit to make it look more coordinated.

Preview of change

[kaaanata]

Thanks for the PR and the preview of the updated website. I agree that the existing text color contrast could indeed be improved. I've just had a chance to check out the newly deployed site and noticed that the pure white text in dark mode indeed has a very high contrast, but it also appears to be slightly harsh on the eyes. This effect might be even more pronounced on screens with high brightness, especially during nighttime use. Moving forward, we will experiment with a few different color options and discuss to finalize a color scheme that balances readability and visual comfort.