I managed to recognise ‘//’ within css files and colour them differently, but it requires a little care.
Find the key/section for ‘comment-block’ in the css.tmLanguage file and duplicate it underneath. Amend this duplicate section to:
<key>comment-block-slash</key>
<dict>
<key>begin</key>
<string>\/\/</string>
<key>captures</key>
<dict>
<key>0</key>
<dict>
<key>name</key>
<string>punctuation.definition.comment.slash.css</string>
</dict>
</dict>
<key>end</key>
<string>\n</string>
<key>name</key>
<string>comment.block.slash.css</string>
</dict>
Then search for the following - it occurs two or three times:
<dict>
<key>include</key>
<string>#comment-block</string>
</dict>
Amend it to the following at each found location:
<dict>
<key>include</key>
<string>#comment-block</string>
</dict>
<dict>
<key>include</key>
<string>#comment-block-slash</string>
</dict>
These *includes *are required, otherwise it will just ignore ‘//’ when found within a properties list - and anywhere else for that matter.
Then add the following section to your chosen theme file:
<dict>
<key>name</key>
<string>Slashes css</string>
<key>scope</key>
<string>source.css comment.block.slash.css</string>
<key>settings</key>
<dict>
<key>background</key>
<string></string>
<key>foreground</key>
<string>#FF0000</string>
</dict>
</dict>
I added it near the top, but after Comment, Variable, etc., scopes.
#FF0000 is red but, when you have it working , you are allowed to change the colour
N.B. Of course, you should copy the files before playing with them…