[i18n] Add context to CSS style vs highlight style (#8510)

Cf. <https://github.com/koreader/koreader/pull/8489#issuecomment-985319434>.

frontend/apps/reader/modules/readerhighlight.lua

@@ -663,7 +663,7 @@ function ReaderHighlight:onShowHighlightDialog(page, index)
                 end,
             },
             {
-                text = _("Style"),
+                text = C_("Highlight", "Style"),
                 callback = function()
                     self:editHighlightStyle(page, index)
                     UIManager:close(self.edit_highlight_dialog)

frontend/apps/reader/modules/readertypeset.lua

@@ -9,11 +9,13 @@ local Notification = require("ui/widget/notification")
 local lfs = require("libs/libkoreader-lfs")
 local optionsutil = require("ui/data/optionsutil")
 local _ = require("gettext")
+local C_ = _.pgettext
 local Screen = require("device").screen
 local T = require("ffi/util").template
 
 local ReaderTypeset = InputContainer:new{
-    css_menu_title = _("Style"),
+    -- @translators This is style in the sense meant by CSS (cascading style sheets), relating to the layout and presentation of the document. See <https://en.wikipedia.org/wiki/CSS> for more information.
+    css_menu_title = C_("CSS", "Style"),
     css = nil,
     internal_css = true,
     unscaled_margins = nil,