open site/default/themes/the_theme-which_you-are_using/less/colors.less with a editor for example Notepad++ or Sublime
and at the very bottom of the colors.less file you can place the above css code 
then save and upload the file again.
Go to Admin Panel/System and clear cache - thats it 
Kind regards, Andy 
Info Update: You can use the color.less file for most of CSS or LESS changes, thats the best place to do such things.
open **site/default/themes/the_theme-which_you-are_using/less/colors.less** with a editor for example **Notepad++** or **Sublime**
and at the **very bottom** of the **colors.less** file you can place the above css code :)
then save and upload the file again.
Go to **Admin Panel/System** and **clear cache** - thats it :)
Kind regards, Andy ;)
**Info Update:** You can use the color.less file for most of CSS or LESS changes, thats the best place to do such things.
edited 3 days ago at 6:56 pm