クラシックなサイトコレクションのページにモダン UI のページがあると統一性がないので、クラシック表示をモダンページに近づけようとういう試み。
/* 全体 */
body,
.ms-core-defaultFont,
# pageStatusBar,
.ms-status-msg,
.js-callout-body,
.ms-core-navigation,
.ms-core-pageTitle,
.ms-core-pageTitle a,
.ms-core-listMenu-verticalBox > .ms-core-listMenu-root > li > .ms-core-listMenu-item,
.ms-core-listMenu-verticalBox > .ms-core-listMenu-root > li > .ms-core-listMenuEdit,
.ms-core-listMenu-heading,
.ms-tv-header {
  font-family: Yu Gothic,Meiryo UI,Meiryo,MS Pgothic,Osaka,Segoe UI,-apple-system,BlinkMacSystemFont,Roboto,Helvetica Neue,sans-serif;
}
.ms-core-listMenu-verticalBox > .ms-core-listMenu-root > li > .ms-core-listMenu-item,
.ms-core-listMenu-verticalBox > .ms-core-listMenu-root > li > .ms-core-listMenuEdit,
.ms-core-listMenu-heading,
.ms-tv-header {
  font-size: 14px;
}
.ms-core-pageTitle, .ms-core-pageTitle a {
  font-weight: 100;
}

/* ヘッダー */
.o365cs-base .o365cs-topnavBGColor,
.o365cs-base .o365cs-topnavBGColor-2,
.o365cs-topnavBGColor.o365cs-base,
.o365cs-topnavBGColor-2.o365cs-base {
  background-color: #000 !important;
}

# siteIcon,
.ms-siteicon-a,
.ms-siteicon-img {
  width: 64px;
  height: 64px;
}

# siteIcon {
  line-height: 64px;
  background-color: #0078D7;
  text-align: center;
  transition: all .5s ease;
  color: #ffffff;
  font-size: 21px;
}

[src*='siteIcon'].ms-siteicon-img {
  display: none;
}

.ms-siteicon-a::after {
  content: "S";
  color: white;
}

# titleAreaBox {
  margin: 10px 32px;
}

# s4-titlerow {
  height: 87px;
  position: fixed;
  top: 50px;
  margin-left: 206px;
}

# s4-workspace {
  top: 156px;
}

# s4-ribbonrow {
  position: fixed;
  top: 165px;
  z-index: 0;
  width: 100%;
  padding-bottom: 20px;
  margin-left: 206px;
}

# globalNavBox, .ms-clientcontrol-chromeheader,
.ms-cui-topBar2,
div.ms-cui-TabRowRight {
  height: 40px;
}
div.ms-cui-TabRowRight {
  line-height: 40px;
  position: fixed;
  top: 67px;
  right: 44px;
}

# fullscreenmodebox {
  display: none;
}

.ms-cui-tabContainer {
  position: relative;
  top: -150px;
}

.ms-cui-ct-ul {
  height: 40px;
}
.ms-cui-ct-ul .ms-cui-tt,
.ms-cui-tts,
.ms-cui-tts-scale-1,
.ms-cui-tts-scale-2 {
  height: 100%;
}
@media screen and (max-width:1024px) {
  .ms-cui-tts,
  .ms-cui-tts-scale-1,
  .ms-cui-tts-scale-2 {
    position: absolute;
    right: 12px;
  }
  .ms-cui-tts-scale-2 .ms-cui-tt-a {
    width: auto;
    padding: 0px 8px;
  }
}

.ms-cui-tt-span,
.ms-cui-ct-ul .ms-cui-tt-a:hover .ms-cui-tt-span {
  padding-bottom: 12px;
}

.ms-cui-tabBody {
  height: auto;
  border-style: none;
  border-width: 0px;
  margin: 4px 0px;
}

.ms-core-listMenu-verticalBox,
.ms-core-listMenu-horizontalBox {
  font-size: 1.1em;
}
# DeltaTopNavigation {
  position: absolute;
  top: 1px;
  left: 32px;
}

.ms-core-listMenu-horizontalBox .ms-core-listMenu-selected:link,
.ms-core-listMenu-horizontalBox .ms-core-listMenu-selected:visited,
.ms-core-listMenu-horizontalBox .ms-core-listMenu-selected,
.ms-tv-selected:link {
  color: #666666;
  font-weight: normal;
}
.ms-core-listMenu-selected:link, .ms-core-listMenu-selected:visited, .ms-core-listMenu-selected {
    border-left: 2px solid #0078D7;
}

/* 検索ボックス */
# searchInputBox {
  margin: 0px;
  position: fixed;
  left: 0px;
  top: 50px;
}
.ms-srch-sb > input {
  height: 34px;
  width: 148px;
  background-color: white;
  margin: 0;
  padding: 0px 2px;
}
.ms-srch-sb > input,
.ms-srch-sb > input:hover,
.ms-srch-sb > input:focus,
[class^="ms-srch-sb"],
[class^="ms-srch-sb"]:hover,
[class^="ms-srch-sb"]:focus {
  border: none;
}

.ms-srch-sb > .ms-srch-sb-searchLink {
  height: 34px;
  width: 34px;
  background-color: #0078D7;
}
.ms-srch-sb-searchImg {
  display: none;
}

.ms-srch-sb-searchLink::before {
  content: "";
  position: absolute;
  top: 13px;
  left: 13px;
  width: 8px;
  height: 8px;
  color: white;
  border-top: 2px solid #FFF;
  border-right: 2px solid #FFF;
  -webkit-transform: rotate(45deg);
  transform: rotate(45deg);
}
.ms-srch-sb-searchLink::after {
  content: "";
  position: absolute;
  background-color: #fff;
  height: 2px;
  width: 13px;
  top: 17px;
  left: 10px;
}

@media screen and (max-width:1024px) {
  #searchInputBox {
    position: absolute;
    left: 3px;
    top: 119px;
  }
}

/* サイドメニュー */
# sideNavBox {
  margin: 0px;
  padding: 12px 0px;
  height: 100%;
  width: 206px;
  position: fixed;
  top: 84px;
  border-top: 1px solid #eaeaea;
  border-right: 1px solid #eaeaea;
}

.ms-core-sideNavBox-removeLeftMargin {
  margin: 0px;
}

@media screen and (max-width:1024px) {
  #s4-titlerow,
  #s4-ribbonrow {
    margin-left: 0px;
  }
  #sideNavBox {
    display: none;
  }
  #contentBox {
    margin: 1% 2% !important;
  }
}