Builder is not retaining Global Theme CSS

Hello,

I am having some trouble getting my Global Theme CSS to stay in Builder. Whenever I paste the following CSS into the Global Theme CSS area in Builder, the CSS applies, works on the front-end, but whenever I close builder and reopen, the Global Theme CSS is reverted back to the previous. If I were to save without pasting the CSS back into the Global Theme CSS area, the CSS would be reverted back and lost. I have validated the CSS to make sure there is nothing goofy. I have tried adding and removing vendor prefixes to see if that is the problem. I have tried with and without the #ufc0 references, but have not had any luck.

Some questions in regards to CSS Vendor Prefixes and Global Theme CSS:
What CSS Vendor Prefixes are needed and allowed in Global CSS? What is causing the following CSS to not work in Builder? What vendor prefixes should not be used? Is there a guide available for the allowed CSS and related for Upfront / Builder? Just to check if something like display: flex; is allowed or not....

Please review and let me know what is causing this issue.

body { background-color: #ffffff; background-color: rgba(255,255,255,1); }
.gbn { width: 100%; height: 3px; background: #ufc0; background: -moz-linear-gradient(left, rgba(255,255,255,0) 0%, #ufc0 50%, rgba(255,255,255,0) 100%); background: -webkit-linear-gradient(left, rgba(255,255,255,0) 0%, #ufc0 50%, rgba(255,255,255,0) 100%); background: linear-gradient(to right, rgba(255,255,255,0) 0%, #ufc0 50%, rgba(255,255,255,0) 100%); }
.gbn2 { width: 100%; height: 3px; background: #ufc0; background: -moz-linear-gradient(left, #ufc0 0%, #ufc0 50%, rgba(255,255,255,0) 100%); background: -webkit-linear-gradient(left, #ufc0 0%, #ufc0 50%, rgba(255,255,255,0) 100%); background: linear-gradient(to right, #ufc0 0%, #ufc0 50%, rgba(255,255,255,0) 100%); }
.gbn3 { width: 100%; height: 3px; background: #ufc0; background: -moz-linear-gradient(left, rgba(255,255,255,0) 0%, #ufc0 50%, #ufc0 100%); background: -webkit-linear-gradient(left, rgba(255,255,255,0) 0%, #ufc0 50%, #ufc0 100%); background: linear-gradient(to right, rgba(255,255,255,0) 0%, #ufc0 50%, #ufc0 100%); }
.gbn0 { width: 100%; height: 3px; background: #000000; background: -webkit-linear-gradient(left, rgba(0,0,0,0) 0%, rgba(0,0,0,1) 50%, rgba(0,0,0,0) 100%); background: -moz-linear-gradient(left, rgba(0,0,0,0) 0%, rgba(0,0,0,1) 50%, rgba(0,0,0,0) 100%); background: linear-gradient(left, rgba(0,0,0,0) 0%, rgba(0,0,0,1) 50%, rgba(0,0,0,0) 100%); }
.desktop-breakpoint .ubermenu-main, .desktop-breakpoint .ubermenu-social { text-align: right; }
.tablet-breakpoint .ubermenu-main, .tablet-breakpoint .ubermenu-social { text-align: center; }
.mobile-breakpoint .ubermenu-main, .mobile-breakpoint .ubermenu-social { text-align: center; }
.desktop-breakpoint .upfront-region-container-roof .upfront-region-container-bg { box-shadow: 0 3px 9px -3px rgba(0,0,0,.5); }
.desktop-breakpoint .upfront-region-container-roof { position: absolute; width: 100%; }
.desktop-breakpoint .upfront-region-intro { padding-top: 120px; }
.desktop-breakpoint .upfront-region-container-intro .upfront-region-container-bg { min-height: 570px; }
.tablet-breakpoint .upfront-region-container-intro .upfront-region-container-bg { min-height: 460px; }
.tablet-breakpoint .upfront-grid-layout { width: calc(100% - 3em); min-width: 540px; }
.mobile-breakpoint .upfront-grid-layout { width: calc(100% - 1em); min-width: 315px; }
.popup .ubermenu-icon { text-shadow: rgba(0,0,0,.25) 1px 1px 0; padding-right: 8px; padding-left: 7px; min-width: 15px; text-align: center; }
.popup .wdpu-content p { line-height: 1.62em; }
.popup .wdpu-text { margin: 0 240px 0 0; }
.popup .wdpu-image { width: 240px; }
.popup .wdpu-image img { max-width: 240px; max-height: 180px; }
.popup .wdpu-msg p { font-size: 18px; padding-left: 5px; }
.member-list { display: -webkit-box; display: -ms-flexbox; display: flex; -ms-flex-wrap: wrap; flex-wrap: wrap; margin-left: -10px; margin-top: -10px; }
.member-list li.member-link { -webkit-box-flex: 1; -ms-flex: 1 0 270px; flex: 1 0 270px; margin-left: 10px; margin-top: 10px; }
.member-list li.member-link { list-style: none; list-style-type: none; cursor: pointer; }
.member-list li.member-link span { display: block; width: 100%; text-decoration: none; line-height: 32px; padding-bottom: 5px; box-sizing: border-box; }
ul.member-list li:after { border-bottom: 2px solid #d4d4d4; display: inline-block; display: block; content: ''; }
ul.member-list li:empty:after { border-bottom: 0 solid #d4d4d4; }
.tablet-breakpoint .member-list li.member-link { max-width: calc(50% - 10px); }
.desktop-breakpoint .member-list li.member-link { min-width: calc(33.33333% - 10px); }
.member-list li.member-link { background: #f9f9fa; color: #000000; box-sizing: border-box; padding: 10px; }
.member-list li.member-link:hover { background-color: #ufc0; color: #FFF; border-radius: 4px; box-shadow: 0 0 3px rgba(0,0,0,.25); }

Thanks,
Michael