Fix syntax issue

  :not(kbd) > kbd,
 ^
      Property "not" must be followed by a ':'
      in /project/src/sass/_theme_rst.sass (line 518, column 3)
    at /project/node_modules/webpack/lib/NormalModule.js:316:20
    at /project/node_modules/loader-runner/lib/LoaderRunner.js:367:11
    at /project/node_modules/loader-runner/lib/LoaderRunner.js:233:18
    at context.callback (/project/node_modules/loader-runner/lib/LoaderRunner.js:111:13)
    at Object.callback (/project/node_modules/sass-loader/dist/index.js:89:7)
    at Object.done [as callback] (/project/node_modules/neo-async/async.js:8069:18)
    at options.error (/project/node_modules/node-sass/lib/index.js:294:32)
 @ multi ./src/theme.js ./src/sass/theme.sass theme[1]
Child mini-css-extract-plugin node_modules/css-loader/dist/cjs.js!node_modules/sass-loader/dist/cjs.js??ref--5-2!src/sass/badge_only.sass:
    Entrypoint mini-css-extract-plugin = *
    [0] ./node_modules/css-loader/dist/cjs.js!./node_modules/sass-loader/dist/cjs.js??ref--5-2!./src/sass/badge_only.sass 4.69 KiB {0} [built]
        + 7 hidden modules
Child mini-css-extract-plugin node_modules/css-loader/dist/cjs.js!node_modules/sass-loader/dist/cjs.js??ref--5-2!src/sass/theme.sass:
    Entrypoint mini-css-extract-plugin = *
    [0] ./node_modules/css-loader/dist/cjs.js!./node_modules/sass-loader/dist/cjs.js??ref--5-2!./src/sass/theme.sass 227 bytes {0} [built] [failed] [1 error]

    ERROR in ./src/sass/theme.sass (./node_modules/css-loader/dist/cjs.js!./node_modules/sass-loader/dist/cjs.js??ref--5-2!./src/sass/theme.sass)
    Module build failed (from ./node_modules/sass-loader/dist/cjs.js):

      :not(kbd) > kbd,
     ^
          Property "not" must be followed by a ':'
          in /project/src/sass/_theme_rst.sass (line 518, column 3)

Author: benjaoming

src/sass/_theme_rst.sass

@@ -515,10 +515,10 @@
     border-radius: $base-line-height / 6
     padding: ($base-line-height / 10) ($base-line-height / 4)
     margin: auto ($base-line-height / 12)
-  :not(kbd) > kbd,
-  :not(kbd) > .kbd,
-  :not(.kbd) > kbd,
-  :not(.kbd) > .kbd
+  &:not(kbd) > kbd,
+  &:not(kbd) > .kbd,
+  &:not(.kbd) > kbd,
+  &:not(.kbd) > .kbd
     color: inherit
     font-size: 80%
     background-color: lighten($text-light, 30%)