add generic text input styling

text inputs are now styled at every occurence in the app,
style input by default, and provide a .mx_textinput class
if buttons need to appear inside the input styling.

Before this was partially done in _common.scss, but as it's
highly theme-dependent, makes more sense to do it in the theme.

res/css/_common.scss

@@ -52,11 +52,6 @@ a:visited {
 
 input[type=text], input[type=password], textarea {
     background-color: transparent;
-    color: $primary-fg-color;
-}
-
-input[type=text].error, input[type=password].error {
-    border: 1px solid $warning-color;
 }
 
 input[type=text]:focus, input[type=password]:focus, textarea:focus {