fix: keep text dark on form controls given a white background

sanitize.css forms.css makes form controls inherit the text color, so
the rules that put them back on white left white text wherever the
surrounding panel sets it, as in the nimgsearch search panel and the
open.neuroinf.jp advanced search.

- set the text to black next to each white control background,
  matching how the browser drew the controls before sanitize.css

src/common/assets/theme/style.css

@@ -317,4 +317,5 @@ ul.mainmenu {
    selector on white. */
 .leftcolumn .blockContent select {
   background-color: #ffffff;
+  color: #000000;
 }

src/database/Database.module.css

@@ -77,4 +77,5 @@
 .database :global(.fieldDate input),
 .database :global(.listTable select) {
     background-color: #ffffff;
+    color: #000000;
 }

src/database/blocks/IndexTree.module.css

@@ -18,6 +18,7 @@
     margin: 3px 3px 10px;
     /* sanitize.css forms.css leaves buttons transparent */
     background-color: #ffffff;
+    color: #000000;
 }
 
 .indexTree div span {