Fix invisible checkboxes in Gruvbox theme (#2135)

* Added Gruvbox Dark color theme

* Correct formatting by running prettier

* Fixed invisible checkboxes and tweaked colors in Gruvbox theme

ui/src/themes/gruvboxDark.css.js

@@ -47,4 +47,8 @@ module.exports = `
     color: #ebdbb2
 }
 
+.MuiCheckbox-colorSecondary.Mui-checked {
+    color: #458588 !important
+}
+
 `

ui/src/themes/gruvboxDark.js

@@ -32,7 +32,7 @@ export default {
     },
     MuiButton: {
       textPrimary: {
-        color: '#8ec07c',
+        color: '#458588',
       },
       textSecondary: {
         color: '#ebdbb2',