[client] Clean up and standardize client CSS

[ix5]

Since we're throwing all user-supplied style overrides overboard with the addition of isso- prefixes in #816 anyway, the upcoming 0.13 release might be a great opportunity to review and rewrite some of the CSS that's shipped on the client side.

As a reference point: This is the current CSS: isso/css/isso.css

What I see as issues:

• Inconsistent colors (seemingly arbitrarily chosen instead of defining and using a palette)
• Inconsistent color syntax (for instance written as either #eee, #EEE, #eeeeee, or rgba(0, 0, 0, 0.1))
• Usage of important!
• Way too specific selectors. They should not be necessary any more since everything is now isso--namespaced
• Mixing of px and em units (use rem instead, see caniuse: rem), also no notion of a grid
• Usage of calc() (bad browser support, see caniuse: calc)
• Current preview gradient is kind of ugly (but has okay browser support, see caniuse: repeating-linear-gradient)
• font-weight given as either numerical (700) or text (bold) value
• No structure to stylesheet and no section headers/explanations
• No resets (which might be a good thing to fit in better with the "host" site)

Related: #842 (comment), usage of Tailwind's color palette.

Wishlist:

• Dark theme via media queries (or at least make dark overrides easier)