Greetings. I wanted to ask about the 5 Mind Green DF theme. Would it be okay to use in v.9.1.2.1? I use the theme currently and I like it but I wanted to upgrade the site (and theme).

I am using the 5mind theme in 9.1.2.1 and (so far) have not had any problems. I also did some extensive mods on it (colors, width, other CSS stuff) and can now have the left side column be red and the right side column blue. Check out www.powwowtime.com in a couple days if you want to see how it turned out.

I opened header.html and changed "mainside" to "leftside"...

That's from:
<!-- start the left cell for blocks -->
<!-- IF S_LEFTBLOCKS -->
<td class="mainside">
to:
<!-- start the left cell for blocks -->
<!-- IF S_LEFTBLOCKS -->
<td class="leftside">

in footer.html I changed "mainside" to"rightside"...

That's from:
<!-- start the right cell for blocks -->
<!-- IF S_RIGHTBLOCKS -->
<td class="mainside">
to:
<!-- start the right cell for blocks -->
<!-- IF S_RIGHTBLOCKS -->
<td class="rightside">

And in style.css I replaced:

.mainside {
font-size: 12px;
vertical-align: top;
padding-left: 4px;
padding-right: 4px;
padding-top: 8px;
color: #dddddd;
background: #990000;
}
.mainside a {
color: #ffcc99;
}
.mainside a:hover{
color: #ff0000;
text-decoration: none;
}

With:

.leftside {
font-size: 12px;
vertical-align: top;
padding-left: 4px;
padding-right: 4px;
padding-top: 8px;
color: #dddddd;
background: #990000;
}
.leftside a {
color: #ffcc99;
}
.leftside a:hover{
color: #ff0000;
text-decoration: none;
}

.rightside {
font-size: 12px;
vertical-align: top;
padding-left: 4px;
padding-right: 4px;
padding-top: 8px;
color: #dddddd;
background: #990000;
}
.rightside a {
color: #ffcc99;
}
.rightside a:hover{
color: #ff0000;
text-decoration: none;
}

In the above examples they have the same colors, but you can change them to have different colors (left vs. right).

I also downloaded the CenterBlock and CenterBlock2 modules, which sort of give a Dragonfly site designer a more Joomla! -like theming experience.