I use ubuntu 10.10 and installed the Eclipse indigo from the official eclipse website. It's a java editor (not CDT) and characters on the content assist popup are not readable.
Is there any solution?
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Let me to cite answer for 12.04
Simply edit the file /usr/share/themes/Ambiance/gtk-2.0/gtkrc search for
base[ACTIVE] = shade (0.94, @bg_color)
change the shade percentage from 0.94 to whatever visible like 0.54
base[ACTIVE] = shade (0.54, @bg_color)
then it works.
Your screen capture looks what I get. It's OK for me but if it's no good on your screen, you should be able to change the colours by going to Window > Preferences > General > Appearance > Colors and Fonts and changing either or both of 'Content Assist background color', 'Content Assist foreground color'. But it doesn't look like you can separately control the colours for highlighted and other lines.
When using the default Ubuntu 12.04 Ambiance colour scheme I changed tooltip background colour to tooltip_bg_color:#7F7F7F allowing inspector popup windows to show visible text:
sudo gedit /usr/share/themes/Ambiance/gtk-2.0/gtkrc gtk-color-scheme = "base_color:#ffffff\nfg_color:#4c4c4c\ntooltip_fg_color:#ffffff\nselected_bg_color:#f07746\nselected_fg_color:#FFFFFF\ntext_color:#3C3C3C\nbg_color:#F2F1F0\ntooltip_bg_color:#7F7F7F\nlink_color:#DD4814"
Log out of ubuntu and log back in to see the effect
For more of a Ubuntu experience try using #DA8C6D as the new background colour.