Use regular expression substitution. Many editors support regular expressions, including Vim.
Here's how to do it from the command line, using sed (Stream EDitor):
sed -i -e "s/.*:.*/&;/" INPUT_FILE.css
Some versions of sed don't support in-place editing (writing the output file to the input file):
sed -e "s/.*:.*/&;/" INPUT_FILE.css > OUTPUT_FILE.css
sed invoke Stream EDitor commmand line tool
-i edit in-place
-e the next string will be the regular expression: s/.*:.*/&;/
INPUT_FILE.css the name of your text file
The regular expression (RegEx) explained, in detail:
s RegEx command indicates substitution
/ RegEx delimiter: separates command and match expression
.* any string followed by...
: a colon character followed by...
.* any string
/ RegEx delimiter: separates match expression and replacement expression
& RegEx back reference, entire string that was matched by match expression
; the semicolon you wish to add
/ RegEx delimiter: ends replacement expression