Sigg, I know you are very busy, and I am patient, so this isn't "must have NOW"; these are suggestions that would improve Syla, but can be filed under "work in progress".
Have the Main Panel automatically detect the client's height and width, and scale to fit. This can be done manually in the style settings, but it looks odd. Why is this a good idea? Because different users have different screen dimensions. Having the code do it for you makes it work correctly all the time the first time. Right now, on my 1980 x 1020, the Main Panel at the bottom uses about 3/4 of my screen's width, and the sides are too narrow to be useful. The other problem with manually adjusting the scale is that the anchors for the Player, Target, ToT, Player Pet, and really every unit frame other than the Party frames do not come along for the ride. If Syla were automatically adjusting the Main Panel scale, it would keep the anchor points.
Turn the viewport on by default on the bottom under the Main Panel, setting its width to your screen's resolution, and the height to the bottom of the Player frame, even with the top of the Main Panel. When I tried this, there was a 1 or 2 pixel difference in the height of the Main Panel and the viewport, which again, looked weird.
The minimap should not be part of the Main Panel, and should be independent. Trying to change the settings for the minimap cluster hid/destroyed my existing Syla Main Panel. These two should be independent of each other.
Next, the 3D portraits need some love. I got a bit confused looking at RDX's settings and saw two different Blizzard 3D portraits: original and new. I'm assuming the new properly uses the CVar gxStereoEnabled for stereoscopic 3D vision. This CVar is 1 if enabled and present, 0 or nil if off or not present. Currently, all unit frame addons (all, not just RDX) have distorted 3D portraits when using steroscopic vision enabled. Very, very badly distorted. Turning this on or off during gameplay forces a graphics reload. There is an old, yet still accurate post (other than the bug is long fixed) with more information here.
Going back to the viewport for a moment, this is more of a RDX suggestion than a Syla suggestion, but when most people turn on the top viewport, it is because they are intending on putting some kind of Broker/LDB display at the top. You could extend this thought to putting it at the bottom of the screen, under the Main Panel... Anyway, it might be a good idea to prompt the user with "Do you want to use the LDB/Broker feed bar?" and if the answer is yes, create it for the user automatically. The reason I keep using some other Broker display is because RDX's system is unnecessarily complex for something so simple.
I also found the settings for click-casting unnecessary complex, and really preferred using a different addon (Clique) because trying to configure Syla's unit frames for click casting almost drove me crazy.
As you can see, I am still keeping an eye on RDX and the themes, and saw you posted a beta update (woo hoo!!) for both RDX and Sitrax. I am not trying to critical, but helpful. As a new user to RDX, I'm trying to provide insight that an experienced user might ignore because they are used to RDX.