Allow color customization through CSS variables in Gaia and Uncover theme

[yhatt]

Allowed color customization through CSS variables, in gaia theme and uncover theme.

Related: #209.

Usage

Gaia theme

---
theme: gaia
style: |
  :root {
    --color-background: #fff !important;
    --color-foreground: #333 !important;
    --color-highlight: #f96 !important;
    --color-dimmed: #888 !important;
  }
---

Uncover theme

---
theme: uncover
style: |
  :root {
    --color-background: #ddd;
    --color-background-code: #ccc;
    --color-background-paginate: rgba(128, 128, 128, 0.05);
    --color-foreground: #345;
    --color-highlight: #99c;
    --color-highlight-hover: #aaf;
    --color-highlight-heading: #99c;
    --color-header: #bbb;
    --color-header-shadow: transparent;
  }
---

!important is required to make custom colors force, against specified color via HTML class like invert and gaia.

ToDo

• [ ] Default theme
  • Marp default theme is depending on github-markdown-css and it is not provided variables-based color defintion as like as GitHub's dark mode. Current invert class has many unique definitions against the current GitHub dark mode. It seems wise to wait for the support of variables used by GitHub.
• Gaia theme
• Uncover theme