Fix invisible table header text (white-on-white contrast bug)

table th switched to background: transparent + text-transform: uppercase
in a past redesign, but a.tooltip (the link wrapping every <th>'s text
across the whole dashboard) still hardcoded color: #FFFFFF from the old
solid-indigo-background header design. With no colored block behind it
anymore, white text sits directly on --page-bg/--content-bg (near-white),
making every table header effectively unreadable.

Switch a.tooltip to color: var(--content-text) (dark, same variable the
th itself already uses) so header text is visible again.

css/pistar-css-mini.php

@@ -660,13 +660,13 @@ a:hover {
 a.tooltip, a.tooltip:link, a.tooltip:visited, a.tooltip:active  {
     text-decoration: none;
     position: relative;
-    color: #FFFFFF;
+    color: var(--content-text);
 }
 
 a.tooltip:hover {
     text-shadow: none;
     text-decoration: none;
-    color: #FFFFFF;
+    color: var(--content-text);
     background: transparent;
 }
 

css/pistar-css.php

@@ -801,13 +801,13 @@ a:hover {
 a.tooltip, a.tooltip:link, a.tooltip:visited, a.tooltip:active  {
     text-decoration: none;
     position: relative;
-    color: #FFFFFF;
+    color: var(--content-text);
 }
 
 a.tooltip:hover {
     text-shadow: none;
     text-decoration: none;
-    color: #FFFFFF;
+    color: var(--content-text);
     background: transparent;
 }