Fix button text colour in theia

Signed-off-by: Tim Etchells <timetchells@ibm.com>

Author: Tim Etchells

dev/res/css/theia.css

@@ -5,7 +5,7 @@
 */
 :root {
     --vscode-button-background: #0F5891;
-    --vscode-button-foreground: #c9cece;
+    --vscode-button-foreground: #ffffff;
     --vscode-editor-foreground: #d4d4d4;
     --vscode-editor-background: #1e1e1e;
     --vscode-sideBarSectionHeader-foreground: #e0e0e0;