addon/Quests/Frames/OptionsFrame.lua

local function CreateCheckbox(parent, frame, text, optionKey)
    local checkbox = CreateFrame("CheckButton", nil, frame, "InterfaceOptionsCheckButtonTemplate")
    checkbox.Text:SetText(text)
    checkbox:SetScript("OnClick", function(self)
        CzechQuestsAddon.options[optionKey] = self:GetChecked()
    end)
    checkbox:SetPoint("TOPLEFT", parent, "BOTTOMLEFT", 0, -8)
    checkbox:SetChecked(CzechQuestsAddon.options[optionKey])
    return checkbox
end

function CzechQuestsAddon:InitializeOptions()
    CzechQuestsAddon.optionsFrame = {};

    if (WOW_PROJECT_ID == WOW_PROJECT_CLASSIC) then
        local frame
        -- Create Frame
        CzechQuestsAddon.optionsFrame = CreateFrame("Frame", "CzechQuestsOptionsPanel", UIParent)
        CzechQuestsAddon.optionsFrame.name = "CzechQuests"

        -- Create frame title
        local title = CzechQuestsAddon.optionsFrame:CreateFontString(nil, "ARTWORK", "GameFontNormalLarge")
        title:SetPoint("TOPLEFT", 16, -16)
        title:SetText("CzechQuests")

        -- Add DEBUG checkbox
        local debugModeCheckbox = CreateCheckbox(title, CzechQuestsAddon.optionsFrame, "Enable DEBUG mode", "DEBUG_MODE")
        debugModeCheckbox:SetPoint("TOPLEFT", title, "BOTTOMLEFT", 16, -16)

        -- Add Dark Mode checkbox
        local darkModeCheckbox = CreateCheckbox(debugModeCheckbox, CzechQuestsAddon.optionsFrame, "Dark mode (require reload)", "DARK_MODE")
        darkModeCheckbox:SetPoint("TOPLEFT", debugModeCheckbox, "BOTTOMLEFT", 0, -4)

        -- Store it into Addon options
        InterfaceOptions_AddCategory(CzechQuestsAddon.optionsFrame)

    else
        CzechQuestsAddon.optionsFrame = Settings.RegisterVerticalLayoutCategory("CzechQuests")

        -- Add DEBUG checkbox
        local debugCheckbox = Settings.RegisterAddOnSetting(
                CzechQuestsAddon.optionsFrame,
                "CzechQuestsAddon__DEBUG_MODE",
                "DEBUG_MODE",
                CzechQuestsAddon.options,
                type(CzechQuestsAddon.options.DEBUG_MODE),
                "Enable DEBUG mode",
                CzechQuestsAddon.options.DEBUG_MODE
        )
        Settings.CreateCheckbox(CzechQuestsAddon.optionsFrame, debugCheckbox)

        -- Add DARK mode checkbox
        local darkModeCheckbox = Settings.RegisterAddOnSetting(
                CzechQuestsAddon.optionsFrame,
                "CzechQuestsAddon__DARK_MODE",
                "DARK_MODE",
                CzechQuestsAddon.options,
                type(CzechQuestsAddon.options.DARK_MODE),
                "Use dark mode",
                CzechQuestsAddon.options.DARK_MODE
        )
        Settings.CreateCheckbox(CzechQuestsAddon.optionsFrame, darkModeCheckbox)

        -- Store it into Addon options
        Settings.RegisterAddOnCategory(CzechQuestsAddon.optionsFrame)
    end
end

-- initialize options frame
CzechQuestsAddon:InitializeOptions()