Turning on a ruler does not show a visible ruler line for the GitHub themes. This change renders rulers using the `canvas.subtle` color. This matches the color used for the `cursorline` and creates a visible ruler that fits the theme.
markup.strikethrough
This is a slight divergence from the VSCode theme, but it's more accurate. https://github.com/primer/github-vscode-theme/issues/111 https://github.com/primer/github-vscode-theme/issues/88
* Add github themes * Increase contrast for hover and completion menus * Use inheritance for the GitHub themes
