Changing pen color and using a theme can cause text to be unreadable

[Dibasic]

Describe the bug
The only unintended behavior here is that changing your pen color back to 'black' locks it at black and isn't overwritten by color schemes.

To Reproduce
Steps to reproduce the behavior:

1. Set pen color to "Blue"
2. Set theme to "Solarized (Dark)"
3. Realize that looks bad, set pen color to "Black" to reset it
4. You now have black on teal instead of the default solarized color.

Expected behavior
Combining these features should do one of two things:

1. Lock out the pen color feature entirely while the theme is not default white.
2. Have the 'Black' pen color instead default to the theme's default text color. Rename 'Black' to 'Black (Default)' or similar.

Web Browser (please complete the following information):

• Firefox 65.0.2

Screenshots

Pen Code

Additional context

[soryy708]

"Invisible" should be white on white, but becomes extra visible on solarized and black.

[Dibasic]

This is where custom properties could be nice. Setting '--defaultTextColor' to be black on the default, white on high-contrast, base0 on Solarized, etc should work

[soryy708]

https://developer.mozilla.org/en-US/docs/Web/CSS/--*#Browser_compatibility

[soryy708]

Fix was not adequate. #67