I am unable to see what I type in the Gedit editor using dark theme.
This is due to the default setting of highlighting the current line.
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As you can see in the images, by default the Higlight current line feature is enabled in Gedit editor on Ubuntu 20.04 LTS (Focal).
Due to that, you can't see what you type in the current line properly, since the font color and highlight color are almost the same.
You can disable Highlight current line in the preferences of Gedit or change the color of the text in Font & Colors settings.
You have two options:
Open the theme file
/usr/share/gtksourceview-4/styles/tango.xml with root access and around the line 50 there should be something like this :
<color name="aluminium1" value="#eeeeec"/>
Change the color to something less harsh like
Use another theme.The default theme when the global appearance is set to dark is tango but you can also use Classic or Oblivion depending on your mood and your preferences.None of them have the problem of unreadable text when a line is highlighted.Just go to Preferences -> Font and colors and change that.
Also you can take a look at this theme repository to find more themes if you want.