Set explicit text color on dialog and buttons to prevent white-on-white with color-scheme dark

[vincentfretin]

Description:

With color-scheme normal or light, you get

but the black text is from the browser default style.
When you switch to color-scheme: dark for example with

<style>
:root {
  color-scheme: dark;
}
</style>

you get white on white

and with inspector open you get

because color come from the body.aframe-inspector-opened rule.

Changes proposed:

• Define text color explicitly for a-dialog and a-button so we have the first image with color-scheme dark.