Update CSS to remove older properties

[saxamaphone69]

Given the new minVer for XT is Chrome 80 and Firefox 74, a lot of these changes just remove legacy/older properties. Some of this stuff was unprefixed 10 years ago.

Might it break compatibility with the 2 people using legacy browsers? Maybe. They can add back what they need in the Custom CSS.

Future thoughts:

• Since Custom Properties are already being used, there is the possibility of getting all the CSS into 1 file.

  :root.yotsuba {
  --main-background-color: #F0E0D6;
  --main-border-color: #D9BFB7;
  --input-border-color: #EA8;
  --header-bar-background-color: rgba(240,224,214,0.98);
  --header-bar-font-size: 9pt;
  --header-bar-font-color: #B86;
  --header-bar-link-color: #800000;
  --backlink-deadlink-color: #00E;
  --quote-color: #789922;
  --inline-background-color: rgba(255, 255, 255, .14);
  --qr-preview-background-color: rgba(0, 0, 0, .15);
  --qr-link-background: linear-gradient(#FFEFE5, #F0E0D6) repeat scroll 0% 0% transparent;
  --qr-link-background-hover: #F0E0D6;
  --qr-link-border-color: rgb(225, 209, 199) rgb(225, 209, 199) rgb(210, 194, 184);
  --menu-font-color: #800000;
  --menu-font-size: 10pt;
  --menu-entry-focused-background: rgba(255, 255, 255, .33);
  --unread-background-color: rgba(240,224,214,0.5);
  --unread-line-color: rgb(255,0,0);
  --quoting-you-color: #F00;
  --watch-thread-link: url("data:image/svg+xml,<svg viewBox='0 0 26 26' preserveAspectRatio='true' xmlns='http://www.w3.org/2000/svg'><path fill='rgb(128,0,0)' d='M24.132,7.971c-2.203-2.205-5.916-2.098-8.25,0.235L15.5,8.588l-0.382-0.382c-2.334-2.333-6.047-2.44-8.25-0.235c-2.204,2.203-2.098,5.916,0.235,8.249l8.396,8.396l8.396-8.396C26.229,13.887,26.336,10.174,24.132,7.971z'/></svg>");
  }
  .dialog {
  border: 1px solid var(--main-border-color);
  display: block;
  background-color: var(--main-background-color);
  }
  .inline {
  border: 1px solid var(--main-border-color);
  background-color: var(--inline-background-color);
  display: table;
  margin: 2px 0;
  }

• If minVer was bumped to Chrome 88 and Firefox 78, that would allow the use of :is(), which could shave off some more lines here and there

[TuxedoTako]

Thanks for the work!