Update color settings for UI elements and completion menu

- Added darker cyan for UI elements in diagnostics.
- Adjusted FloatBorder and Pmenu colors for better visibility.

lua/settings.lua

@@ -40,6 +40,12 @@ vim.opt.showtabline = 1 -- Show tabline only when there are 2+ tabs
 -- Completion UI for nvim-cmp
 vim.opt.completeopt = { "menu", "menuone", "noselect" }
 
+-- Built-in completion popup style (for Ctrl-X Ctrl-N/F completions)
+-- Use floating window with border for native completion
+vim.opt.pumblend = 0 -- No transparency
+-- Note: Native completion (pumvisible) doesn't support custom borders like LSP floats
+-- The Pmenu* highlight groups control its appearance
+
 -- Spelling
 vim.opt.spelllang = { "en_gb" }