Set ui.virtual.ruler background for GitHub themes (#9487)

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.

runtime/themes/github_dark.toml

@@ -60,6 +60,7 @@ label = "scale.red.3"
 "ui.text.focus" = { fg = "fg.default" }
 "ui.text.inactive" = "fg.subtle"
 "ui.virtual" = { fg = "scale.gray.6" }
+"ui.virtual.ruler" = { bg = "canvas.subtle" }
 
 "ui.selection" = { bg = "scale.blue.8" }
 "ui.selection.primary" = { bg = "scale.blue.7" }

runtime/themes/github_light.toml

@@ -60,6 +60,7 @@ label = "scale.red.5"
 "ui.text.focus" = { fg = "fg.default" }
 "ui.text.inactive" = "fg.subtle"
 "ui.virtual" = { fg = "scale.gray.2" }
+"ui.virtual.ruler" = { bg = "canvas.subtle" }
 
 "ui.selection" = { bg = "scale.blue.0" }
 "ui.selection.primary" = { bg = "scale.blue.1" }