The trick here is that the tabs themselves are not actually a part of the pages - so @████████ example of formatting the page itself is going to be the easiest to implement.

I wanted to try to see if something was possible, and I found a (somewhat complex) option that is at the very least interesting.  You can get CSS applied to the tabs that changes how they appear.  The trick is there doesn't appear to be a way to manipulate the classes that are assigned to the tabs.  What you can manipulate, via the LFForm interface and Javascript, is the titles of the tabs.  So I figured I would try to have the CSS based on the titles.

I made a form with five tabs, just naming them Page 1, Page 2, Page 3, Page 4, and Page 5.  On each tab I added radio buttons that can be marked Complete, and a number field that determines how many of those are not complete.  It's just a simple formula that adds a 0 if they are complete or a 1 otherwise, like this: 

=IF(page2_radio_1="Complete",0,1)+IF(page2_radio_2="Complete",0,1)+IF(page2_radio_3="Complete",0,1)

Then I made a Javascript that checks for changes to the number fields counting up how many radio button fields are not marked complete.  That Javascript renames the page title based on whether or not there are incomplete radio buttons.  It appends either a ✔ character to the end of the page name if all of them are complete or a ⚠ character to the end of the page name if some are not complete.  The Javascript looks like this: 

//Function to update the Page 1 title based on the count field on that page.
LFForm.onFieldChange(function() {
  let page1 = LFForm.getFieldValues({variableName: "page1_count"});
  if(page1 != 0) {
    LFForm.changeFormSettings({pagination: [{pageId: 0, label: "Page 1 ⚠"}]});
  }
  else {
    LFForm.changeFormSettings({pagination: [{pageId: 0, label: "Page 1 ✔"}]});
  }
}, {variableName: "page1_count"});

//Function to update the Page 2 title based on the count field on that page.
LFForm.onFieldChange(function() {
  let page2 = LFForm.getFieldValues({variableName: "page2_count"});
  if(page2 != 0) {
    LFForm.changeFormSettings({pagination: [{pageId: 1, label: "Page 2 ⚠"}]});
  }
  else {
    LFForm.changeFormSettings({pagination: [{pageId: 1, label: "Page 2 ✔"}]});
  }
}, {variableName: "page2_count"});

//Function to update the Page 3 title based on the count field on that page.
LFForm.onFieldChange(function() {
  let page3 = LFForm.getFieldValues({variableName: "page3_count"});
  if(page3 != 0) {
    LFForm.changeFormSettings({pagination: [{pageId: 2, label: "Page 3 ⚠"}]});
  }
  else {
    LFForm.changeFormSettings({pagination: [{pageId: 2, label: "Page 3 ✔"}]});
  }
}, {variableName: "page3_count"});

//Function to update the Page 4 title based on the count field on that page.
LFForm.onFieldChange(function() {
  let page4 = LFForm.getFieldValues({variableName: "page4_count"});
  if(page4 != 0) {
    LFForm.changeFormSettings({pagination: [{pageId: 3, label: "Page 4 ⚠"}]});
  }
  else {
    LFForm.changeFormSettings({pagination: [{pageId: 3, label: "Page 4 ✔"}]});
  }
}, {variableName: "page4_count"});

//Function to update the Page 5 title based on the count field on that page.
LFForm.onFieldChange(function() {
  let page5 = LFForm.getFieldValues({variableName: "page5_count"});
  if(page5 != 0) {
    LFForm.changeFormSettings({pagination: [{pageId: 4, label: "Page 5 ⚠"}]});
  }
  else {
    LFForm.changeFormSettings({pagination: [{pageId: 4, label: "Page 5 ✔"}]});
  }
}, {variableName: "page5_count"});

Finally, I added some CSS that looks at the tabs, and applies formatting based on whether the title contains the ✔ character or the ⚠ character. 

//Format the complete tabs.
&.Form .form-tab[title~="✔"],
&.Form .form-tab.active[title~="✔"]
{
  background-color: #00FF00;
}

//Format the incomplete tabs.
&.Form .form-tab[title~="⚠"],
&.Form .form-tab.active[title~="⚠"]
{
  background-color: #FF0000;
  color: #FFFFFF;
}

The end result looks like this:

Its not perfect but it is an interesting start at least.