Fix light theme colours

[noisysocks]

Simplifies the CSS somewhat by removing some redundant CSS rules, and tweaks the light theme so that:

• The icon in the upper right is grey, matches Gutenberg, and darkens on hover. Fixes #116
• The border underneath the header is a lighter grey that matches Gutenberg. Fixes #123
• The header text is a lighter grey that matches Gutenberg
• The body text is a darker grey that matches Gutenberg

I haven't touched dark mode which is tougher to tweak since there's no dark Gutenberg to copy 🙂