Could you please use a mono-type font for names and code. it would be easier to read.

and BTW the background from the text must not be bright as a textbox. make it darker like a label.

some code examples:

data:extend({
    {
        type = "font",
        name = "Kux-GuiLab-mono-12",
        from = "Kux-GuiLab-mono", --> locale/en/info.json
        size = 12
    },
})
local styles = data.raw["gui-style"].default
styles["kux-reader_textbox"] = {
    height = 500,
    maximal_width = 1200,
    minimal_width = 600,
    parent = "textbox",
    type = "textbox_style"
}

styles["reader_textbox_mono"] = {
    type   = "textbox_style",
    parent = "kux-reader_textbox",
    font   = "Kux-GuiLab-mono-12" --> data.raw.font[...]
}

styles["kux-label-textbox"] =
{
    type = "textbox_style",
    active_background = {},
    bottom_padding = 4,
    default_background = {},
    disabled_background = {},
    disabled_font_color = {a = 0.5,b = 0.5,g = 0.5,r = 0.5},
    font_color = {241,241,241},
    padding = 8,
    rich_text_setting = "disabled",
    selection_background_color = {0,120,212},
    top_padding = 4,
    --vertically_stretchable = "on",
    horizontally_stretchable = "on",
    maximal_width = 0,
    font   = "Kux-GuiLab-mono-12" --> data.raw.font[...]
}

locale/en/info.json

{
    "language-name": "English",
    "font": {
        "Kux-GuiLab-mono":
        [
        "__core__/fonts/NotoMono-Regular.ttf",
        "__core__/fonts/NotoSans-Regular.ttf",
        "__core__/fonts/NotoSansCJKtc-Regular.ttf",
        "__core__/fonts/NotoSansThai-Regular.ttf",
        "__core__/fonts/NotoSansArabic-Regular.ttf",
        "__core__/fonts/NotoSansHebrew-Regular.ttf",
        "__core__/fonts/DejaVuSansMono.ttf"
        ]
    }
}

I think this requirement is not a high priority

well if you had to go with my eyes then yes ;-)
it was just a suggestion. and as always my ideas are just suggestions for improvements, do it if you have the time, desire and mood for it
thx for your time