theme support

[lino-levan]

It would be cool to support configurable themes and given that we are using CSS variables, it shouldn't be too difficult.

I'm thinking that this can either be done by (A) setting individual variables in pyro.yml, (B) through a url, or maybe both.

A:

# ... other stuff
theme:
  canvas-default: rgb(255,0,0)

B:

theme: https://deno.land/x/pyro_themes/solenoid.yml

These are the current variables:

  --color-canvas-default-transparent:rgba(255,255,255,0);
  --color-prettylights-syntax-comment:#6e7781;
  --color-prettylights-syntax-constant:#0550ae;
  --color-prettylights-syntax-entity:#8250df;
  --color-prettylights-syntax-storage-modifier-import:#24292f;
  --color-prettylights-syntax-entity-tag:#116329;
  --color-prettylights-syntax-keyword:#cf222e;
  --color-prettylights-syntax-string:#0a3069;
  --color-prettylights-syntax-variable:#953800;
  --color-prettylights-syntax-string-regexp:#116329;
  --color-prettylights-syntax-markup-deleted-text:#82071e;
  --color-prettylights-syntax-markup-deleted-bg:#ffebe9;
  --color-prettylights-syntax-markup-inserted-text:#116329;
  --color-prettylights-syntax-markup-inserted-bg:#dafbe1;
  --color-prettylights-syntax-constant-other-reference-link:#0a3069;
  --color-fg-default:#24292f;
  --color-fg-muted:#57606a;
  --color-canvas-default:#fff;
  --color-canvas-subtle:#f6f8fa;
  --color-border-default:#d0d7de;
  --color-border-muted:#d8dee4;
  --color-neutral-muted:rgba(175,184,193,.2);
  --color-accent-fg:#0969da;
  --color-accent-emphasis:#0969da;
  --color-danger-fg:#cf222e