Dark mode input fix (fixes #13312) (#13414)

* fix: add form-control class where missing

* fix: chat input background colour

* fix(pr): don't commit package changes

* fix(css): use white variable

Co-authored-by: J Sanderson <julia@parentscheme.com>
Co-authored-by: Sabe Jones <sabrecat@gmail.com>

website/client/src/components/challenges/challengeModal.vue

@@ -154,6 +154,7 @@
         <input
           v-model="workingChallenge.prize"
           type="number"
+          class="form-control"
           :min="minPrize"
           :max="maxPrize"
           @change="enableSubmit"