Element picker text area has unreadable text

[Sesshu]

Prerequisites

• I verified that this is not a filter issue
  • Filter issues MUST be reported at filter issue tracker, NOT here.
• I performed a cursory search of the issue tracker to avoid opening a duplicate issue
  • Your issue may already be reported.
• I tried to reproduce the issue when...
  • uBlock Origin is the only extension
  • uBlock Origin with default lists/settings
  • using a new, unmodified browser profile
• I am running the latest version of uBlock Origin
• I checked the documentation to understand that the issue I report is not a normal behavior

Description

When using a system theme that uses a bright text color (e.g. white) the text inside the element pickers text area becomes illegible because the background-color of the text area is set to white by this addon's CSS.

A specific URL where the issue occurs

https://www.mozilla.org

Steps to Reproduce

1. Install a system theme that changes text color to white (e.g. Adwaita-dark from Gnome and start Firefox with MOZ_ALLOW_GTK_DARK_THEME=true firefox )
2. Use uBlock to enter element picker mode and click on somewhere on a web page to open up the element picker dialog.

Expected behavior:

The text inside the text area with the rule that the user is about to create is readable.

Actual behavior:

The text is not readable.

Your environment

• uBlock Origin version: uBlock Origin v1.15.24
• Browser Name and version: Firefox 59.0.2
• Operating System and version: ArchLinux (No version numbering)

Possible fix

Either remove the background-color CSS rule to have the text area use the system default background color or add a color rule. I've attached a patch that does the latter and fixes this issue.

epicker.patch

[gwarser]

Mint KDE, GTK3 Theme Breeze-Dark, Firefox Nightly, UBO dev,
MOZ_ALLOW_GTK_DARK_THEME=true not required.