Hi,
I don’t know if it’s possible to edit the original theme directly, but a fairly simple alternative would be to create a copy of the original color scheme, modify this copy and then use the modified version.
You can download a copy of the original theme here and save it to {SUBLIME TEXT FOLDER}/Packages/User/Color Schemes/ (create this path if it doesn’t exist yet).
Then, open the file and edit the selection value (line 22) to be whatever you would like it to be.
Finally, choose this theme by going to Preferences > Color Scheme > User > Color Schemes and choosing Solarized (Dark) - Modified.
Obs 1: You may need to restart Sublime Text to see the changes.
Obs 2: {SUBLIME TEXT FOLDER} will vary according to your OS. See here.