If you enable the checkbox Show whitespace characters under General → Editors → Text Editors, the characters are a
bit too offensive. Sadly, there's no option to change the transparency of them. The default (and fixed) value is
255d (ca. 40% visible / 60% transparency). You can only change the value using a hex-editor:
org.eclipse.jface.text_3.3.1r331_v20070629.jarin the plugins folder of Eclipse using an archiver (e.g. Total Commander)
jump to the location
0x1769 and find the following lines:
001750: 0a 2a b4 01 1f b6 01 3c 36 0a 2a b4 01 1d 99 00 001760: 21 2b b6 01 4e 36 0b 2b 10>64<b6 01 51 2a 2b 15 001770: 09 15 0a b7 01 38 2b 15 0b b6 01 51 a7 00 0c 2a
64 is the value described above.
00h = white/transparent,
FFh = black.)
33h would be 80% transparency and is a nice value.
After saving your changes, put the file back into the .jar and run Eclipse.