Improve letter spacing on Black background

Text on black background looks congested. Adding more space something around `0.05rem` will help improve readability.

Now:

With `0.05rem` letter spacing:

Inspect the matrix.org website styles for the black-background text shown in the issue and identify the relevant selector or stylesheet. Compare the current rendering with the screenshots, apply the requested letter-spacing change, and verify readability across the affected text.

Written by the indexing model from the issue text.