feat: support toolbar icon accent color

[jack-mil]

Introduced in Notepad++ v8.8. Latte will use mono-color icons, for contrast reasons. We are abusing Npp "Dark Mode" feature for a light mode color scheme.

This commit introduces blue accent by default. If desired (I think I will try to create a whiskers template), this can be expanded for options of all accents across all flavors.

• docs: more detailed UI settings description Add information about new UI options that users might optionally want to enable.

• docs: clarify behaviour on older versions When testing, I didn't get any errors, but Notepad++ did overwrite the config file (like it does everytime) with only the recognized options.

Closes #71

Pictures:

(Orange accent for demonstration only)

[jack-mil]

Somehow forgot that the de-facto color is mauve.