Beta 3: default editor color is totally unusable!

Unfortunately the attachment function seems to be broken, I can't add an image of the problem.

The problem is, the SQL editor displays all text, that is not covered by one of the options under "Syntax highlighting", in a really ugly light blue color (theme "Sky").
There seems to be no way to change it. If I select another theme the color will change to something else, but it's still something I cannot select.
If I disable Syntax Highlighting in the preferences, the whole text in the SQL editor is displayed in that color.

How do I change this color?

This is indeed a known issue:

https://forums.allroundautomations.com/ubb/ubbthreads.php?ubb=showflat&Number=63268#Post63268

Unfortunately, the fix described in your post does NOT fix my problem.
It is not the keywords, comments, strings or any of the other named items that are displayed in light cyan, but the rest which is not covered by any of the Syntax Highlighting settings.

Example:

SELECT * FROM tablename WHERE col = 'something'

In the text above "tablenane" and "col" are displayed in light cyan, the rest has the color as selected in Syntax Highlighting settings.
If I remove the checkmark in front of "Keywords" the words "SELECT", "FROM" and "WHERE" become light cyan, too.
But nowhere can I change that default color to something else, or at least I did not find out how to do it, yet.

You should not check or uncheck the checkboxes. You should click on the green indicator to the left of the preference section, with the "System default" hint. Now the colors will revert to the (correct) default settings.

You may need to do the same for the Browser, Grid and Editor fonts on the Preferences > Fonts page.

I managed to partly solve the problem. It required deleting all old settings from "C:\Users\<username>\AppData\Roaming\PLSQL Developer 14" and "C:\Users\<username>\AppData\Roaming\PLSQL Developer 15" and starting completely fresh.
The text in question is now at least black and no longer light cyan.
However, there is still no option to set the default color for the editor text...

To fix all colors, you can do the following:

Download this light.zip file.
Exit PL/SQL Developer.
Go to %APPDATA%\PLSQL Developer 15\Preferences\Themes
(e.g. C:\Users\<your user name>\AppData\Roaming\PLSQL Developer 15\Preferences\Themes)
Unzip the downloaded light.ini file to replace the existing file.
Restart PL/SQL Developer.

Frank, I believe if you go into Preferences\User Interface\Fonts then under the Editor section click the Select button, then select the colour from the drop down list - that is the main editor text colour (for any text not covered by the Syntax Highlighting settings)

Thanks @WexSoft! I missed this setting in the font selection dialog completely.