my-nvim-setup/lua/telescope/_extensions/themes.lua

-- This file can be loaded as a telescope extension
local M = {}

-- reload themes without restarting vim
-- if no theme name given then reload the current theme
M.reload_theme = function(theme_name)
    local reload_plugin = require("utils").reload_plugin

    -- if theme name is empty or nil, then reload the current theme
    if (theme_name == nil or theme_name == "") then
        theme_name = vim.g.nvchad_theme
    end

    if not pcall(require, "themes/" .. theme_name) then
        print("No such theme ( " .. theme_name .. " )")
        return false
    end

    vim.g.nvchad_theme = theme_name

    -- reload the base16 theme
    local ok, base16 = pcall(require, "base16")
    if not ok then
        print("Error: Cannot load base16 plugin!")
        return false
    end
    base16(base16.themes[theme_name], true)

    if
        not reload_plugin {
            "highlights",
            "plugins.bufferline",
            "galaxyline",
            "plugins.statusline"
        }
     then
        print "Error: Not able to reload all plugins."
        return false
    end

    -- now send the provider info to actual refresh
    require("galaxyline.provider").async_load_providers:send()

    return true
    -- open a buffer and close it to reload the statusline
    -- vim.cmd("new|bwipeout")
    -- commented out here as it will not work with telescope picker
end

-- Custom theme picker
-- Most of the code is copied from telescope colorscheme plugin, mostly for preview creation
M.theme_switcher = function(opts)
    local pickers, finders, previewers, actions, action_state, utils, conf
    if pcall(require, "telescope") then
        pickers = require "telescope.pickers"
        finders = require "telescope.finders"
        previewers = require "telescope.previewers"

        actions = require "telescope.actions"
        action_state = require "telescope.actions.state"
        utils = require "telescope.utils"
        conf = require("telescope.config").values
    else
        error "Cannot find telescope!"
    end

    local local_utils = require "utils"
    local reload_theme = M.reload_theme

    -- get a table of available themes
    local themes = local_utils.list_themes()
    if next(themes) ~= nil then
        -- save this to use it for later to restore if theme not changed
        local current_theme = vim.g.nvchad_theme
        local new_theme = ""
        local change = false

        -- buffer number and name
        local bufnr = vim.api.nvim_get_current_buf()
        local bufname = vim.api.nvim_buf_get_name(bufnr)

        local previewer

        -- in case its not a normal buffer
        if vim.fn.buflisted(bufnr) ~= 1 then
            local deleted = false
            local function del_win(win_id)
                if win_id and vim.api.nvim_win_is_valid(win_id) then
                    utils.buf_delete(vim.api.nvim_win_get_buf(win_id))
                    pcall(vim.api.nvim_win_close, win_id, true)
                end
            end

            previewer =
                previewers.new {
                preview_fn = function(_, entry, status)
                    if not deleted then
                        deleted = true
                        del_win(status.preview_win)
                        del_win(status.preview_border_win)
                    end
                    reload_theme(entry.value)
                end
            }
        else
            -- show current buffer content in previewer
            previewer =
                previewers.new_buffer_previewer {
                get_buffer_by_name = function()
                    return bufname
                end,
                define_preview = function(self, entry)
                    if vim.loop.fs_stat(bufname) then
                        conf.buffer_previewer_maker(bufname, self.state.bufnr, {bufname = self.state.bufname})
                    else
                        local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
                        vim.api.nvim_buf_set_lines(self.state.bufnr, 0, -1, false, lines)
                    end
                    reload_theme(entry.value)
                end
            }
        end

        local picker =
            pickers.new(
            {
                prompt_title = "Set NvChad color",
                finder = finders.new_table(themes),
                previewer = previewer,
                sorter = conf.generic_sorter(opts),
                attach_mappings = function()
                    actions.select_default:replace(
                        -- if a entry is selected, change current_theme to that
                        function(prompt_bufnr)
                            local selection = action_state.get_selected_entry()
                            new_theme = selection.value
                            change = true
                            actions.close(prompt_bufnr)
                        end
                    )
                    return true
                end
            }
        )

        -- rewrite picker.close_windows
        local close_windows = picker.close_windows
        picker.close_windows = function(status)
            close_windows(status)
            -- now apply the theme, if success, then ask for default theme change
            local final_theme
            if change then
                final_theme = new_theme
            else
                final_theme = current_theme
            end

            if reload_theme(final_theme) then
                if change then
                    -- ask for confirmation to set as default theme
                    local ans = string.lower(vim.fn.input("Set " .. new_theme .. " as default theme ? [y/N] ")) == "y"
                    local_utils.clear_cmdline()
                    if ans then
                        local_utils.change_theme(current_theme, final_theme)
                    else
                        -- will be used in restoring nvchad theme var
                        final_theme = current_theme
                    end
                end
                -- open a buffer and close it to reload the statusline
                vim.cmd("new|bwipeout")
            else
                final_theme = current_theme
            end
            -- set nvchad_theme global var
            vim.g.nvchad_theme = final_theme
        end
        -- launch the telescope picker
        picker:find()
    else
        print("No themes found in " .. themes_folder)
    end
end

-- register theme swticher as themes to telescope
local present, telescope = pcall(require, "telescope")
if present then
    return telescope.register_extension {
        exports = {
            themes = M.theme_switcher
        }
    }
else
    error "Cannot find telescope!"
end