Add MUG-specific theme styling: variables.less and overrides.less

[lpandath]

create a MUG-specific overrides.less in themes/MUG/ with only the MUG-specific changes, then copy it in the Dockerfile.

note: why do we write overrides? because needed for global override in overrides.less that applies to all inputs, not just the header search.
only:

/* Global input focus color override - MUG green instead of default blue */
.ui.input > input:focus,
.ui.form input[type]:focus {
  border-color: @mugSearch !important;
}

.ui.input > input:hover {
  border-color: @mugSearch !important;
}

note: the Dockerfile copies the entire overrides.less file from themes/MUG/ to replace the one in the installed package
note: used the same styling of format as in invenio-override for variables.less

https://github.com/orgs/sharedRDM/projects/10?pane=issue&itemId=145124734&issue=sharedRDM%7Cmug-project-management%7C32