feat: Allow customizing preview window background

Closes #64

lua/symbols-outline/config.lua

@@ -11,6 +11,7 @@ M.defaults = {
     show_numbers = false,
     show_relative_numbers = false,
     show_symbol_details = true,
+    preview_bg_highlight = 'Pmenu',
     keymaps = { -- These keymaps can be a string or a table for multiple keys
         close = {"<Esc>", "q"},
         goto_location = "<Cr>",

lua/symbols-outline/preview.lua

@@ -136,6 +136,12 @@ local function setup_hover_buf()
     update_hover()
 end
 
+local function set_bg_hl()
+    local winhi = "Normal:" .. config.options.preview_bg_highlight
+    vim.api.nvim_win_set_option(state.preview_win, "winhighlight", winhi)
+    vim.api.nvim_win_set_option(state.hover_win, "winhighlight", winhi)
+end
+
 local function show_preview()
     if state.preview_win == nil and state.preview_buf == nil then
         state.preview_buf = vim.api.nvim_create_buf(false, true)
@@ -195,6 +201,7 @@ function M.show()
 
     show_preview()
     show_hover()
+    set_bg_hl()
 end
 
 function M.close()