Add below in .tmTheme
file
<dict>
<key>name</key>
<string>Function call</string>
<key>scope</key>
<string>meta.function-call variable.function</string>
<key>settings</key>
<dict>
<key>fontStyle</key>
<string></string>
<key>foreground</key>
<string>#A6E22E</string>
</dict>
</dict>
Refs: