.ExistingMemberConsent {
  width: 600px;
  text-align: center;
}

.ExistingMemberConsent--opt-in-reason {
  font-size: 22px;
  line-height: 1.3em;
  margin-bottom: 1em;
}

.ExistingMemberConsent--accept.Button-root,
.ExistingMemberConsent--decline.Button-root {
  max-width: 300px;
  margin: 20px auto;
}

.ExistingMemberConsent--decline.Button-root {
  background: transparent;
  border: none;
  color: #00c0cf;
  text-transform: none;
  height: auto;
  width: auto;
  font-weight: normal;
}

.ExistingMemberConsent--decline.Button-root:hover {
  background: transparent;
  border: none;
  text-decoration: underline;
}

.ExistingMemberConsent--decline.Button-root:active {
  box-shadow: none;
}

.ExistingMemberConsent--how-to-opt-out {
  font-size: 0.9em;
}

.ExistingMemberConsent {
  width: 600px;
  text-align: center;
}

.ExistingMemberConsent--opt-in-reason {
  font-size: 22px;
  line-height: 1.3em;
  margin-bottom: 1em;
}

.ExistingMemberConsent--accept.Button-root,
.ExistingMemberConsent--decline.Button-root {
  max-width: 300px;
  margin: 20px auto;
}

.ExistingMemberConsent--decline.Button-root {
  background: transparent;
  border: none;
  color: #00c0cf;
  text-transform: none;
  height: auto;
  width: auto;
  font-weight: normal;
}

.ExistingMemberConsent--decline.Button-root:hover {
  background: transparent;
  border: none;
  text-decoration: underline;
}

.ExistingMemberConsent--decline.Button-root:active {
  box-shadow: none;
}

.ExistingMemberConsent--how-to-opt-out {
  font-size: 0.9em;
}

@media(max-width: 700px) {
  .ExistingMemberConsent {
    width: 100%;
  }
}

Change width unit to % when <= 700px
Change width unit to % when <= 700px
