"Theme" Option for Opacity for backgrond in UI elements.

[TomLewis]

Something was bugging me and it just clocked what it is, when you open a UI item, it's fully opaque.
Dynmap actually has opacity with the player UI, so you can see what's behind it.

Can we please get a "theme" variable for how opaque we want the pop out UI background?

[JLyne]

With the latest commit this is now possible by changing the --background-base variable to a semi-transparent value (such as dynmap's rgba(0, 0, 0, 0.6).

You may also want to set --text-hover to a different value (such as #222222) to improve contrast on hovered buttons if you choose to do this.