Embarrassing question on setting highlight in `setup()`-call

[PhilippFeO]

I use preservim/vim-markdown and have the following code in my ftplugin/markdown.lua file:

local colors = require("kanagawa.colors").setup()
local cp = colors.palette
local highlight = vim.api.nvim_set_hl

highlight(0, "htmlBold", { fg = cp.autumnRed, bold = true })

This works as expected.

When I use the followin line in the setup()-call

    overrides = function(colors)
        local theme = colors.theme
        return {
            htmlBold = { fg = theme.autumnRed, bold = true },
        ...
        }
    end
}

the highlight doesn't apply. I haven't found any hints on where to set my own highlights except in the overrides section. The highlight of other highlight groups, fi. the Telescope.* ones from your examples works well. What am I doing wrong?