Are there any plans on giving Krita a small face lift with a bit of a more modern theme?

What do you mean with theme? The colors? The shape of the buttons? Or the icons?

But in general, no, there are no plans. If you feel somethings needs to look "more modern" (whatever that might mean) you'll have to roll up your sleeves and start working on it.

The way that bar just goes all the way out of the screen on the left:



The way the borders on these tabs just end like that:



The way these icons look, they have unnecessary boxes around them:



The arrows on the scrollbars aren't centered:



This icon doesn't fit with the rest of the program:



Yes, these are all very small nitpicks but then again I was just wondering if there was ever a plan to make little improvements to the UI, I'm not telling anyone to do it and I certainly don't know how to do it myself.
Also, sorry for not being clear at first.

Some of the little tweaks are on this image that I edited:



This is just some ideas, my intention is not to want Krita to be exacly like it is on that image.
Also that image is imcomplete I didn't edit all arrows of all the scroll bars and I didn't edit every button.
And just so that no one gets me wrong, I love Krita and I've been using it for the past 4 years.

Well, I'm afraid that all the little niggles you show here basically are beyond our ability to change. Those things, like the icon for closing a tab, are part of the widget style and the Qt platform, and both are third party libraries.

Oh alright I didn't know, well thanks for your time and I'm looking foward to keep using Krita.

It is sometimes really difficult, and for someone who isn't deep in the code, actually impossible to see how the parts fit together, and what it is we code, and what we don't code.