feat(spec): support custom highlight blending

Example:

```
{
	highlight_groups = {
		StatusLine = { bg = 'love', blend = 10 }
	}
}
```

lua/rose-pine/theme.lua

@@ -1,5 +1,3 @@
-local blend = require('rose-pine.util').blend
-
 local M = {}
 
 function M.get(config)
@@ -29,10 +27,10 @@ function M.get(config)
 		CursorLineNr = { fg = p.text },
 		DarkenedPanel = { bg = groups.panel },
 		DarkenedStatusline = { bg = groups.panel },
-		DiffAdd = { bg = blend(groups.git_add, groups.background, 0.2) },
+		DiffAdd = { bg = groups.git_add, blend = 20 },
 		DiffChange = { bg = p.overlay },
-		DiffDelete = { bg = blend(groups.git_delete, groups.background, 0.2) },
-		DiffText = { bg = blend(groups.git_text, groups.background, 0.2) },
+		DiffDelete = { bg = groups.git_delete, blend = 20 },
+		DiffText = { bg = groups.git_text, blend = 20 },
 		diffAdded = { link = 'DiffAdd' },
 		diffChanged = { link = 'DiffChange' },
 		diffRemoved = { link = 'DiffDelete' },

lua/rose-pine/util.lua

@@ -33,11 +33,11 @@ end
 ---@param bg string background color
 ---@param alpha number number between 0 (background) and 1 (foreground)
 util.blend = function(fg, bg, alpha)
-	fg = rgb(parse_color(fg))
-	bg = rgb(parse_color(bg))
+	local fg_rgb = rgb(parse_color(fg))
+	local bg_rgb = rgb(parse_color(bg))
 
 	local function blend_channel(i)
-		local ret = (alpha * fg[i] + ((1 - alpha) * bg[i]))
+		local ret = (alpha * fg_rgb[i] + ((1 - alpha) * bg_rgb[i]))
 		return math.floor(math.min(math.max(0, ret), 255) + 0.5)
 	end
 
@@ -50,10 +50,19 @@ util.blend = function(fg, bg, alpha)
 end
 
 ---@param group string
----@param color table<string, string>
+---@param color table<string, any>
 util.highlight = function(group, color)
 	local fg = color.fg and parse_color(color.fg) or 'none'
 	local bg = color.bg and parse_color(color.bg) or 'none'
+
+	if
+		color.blend ~= nil
+		and (color.blend >= 0 or color.blend <= 100)
+		and bg ~= nil
+	then
+		bg = util.blend(bg, parse_color('base') or '', color.blend / 100)
+	end
+
 	local sp = color.sp and parse_color(color.sp) or ''
 
 	color = vim.tbl_extend('force', color, { fg = fg, bg = bg, sp = sp })

readme.md

@@ -85,7 +85,11 @@ require('rose-pine').setup({
 
 	-- Change specific vim highlight groups
 	highlight_groups = {
-		ColorColumn = { bg = 'rose' }
+		ColorColumn = { bg = 'rose' },
+
+		-- Blend colours against the "base" background
+		CursorLine = { bg = 'foam', blend = 10 },
+		StatusLine = { fg = 'love', bg = 'love', blend = 10 },
 	}
 })